When you get to the end of this your going to flip!!!!
I've had the missfire, and power loss issue along with a P300 code on my 2000 blazer, 4.3 Vin W. since it's purchase a year or so ago which required a replacement motor.
I'll try to be quick--- yah right!
List of parts I've replaced---
New parts---
Wires, rotor button, and cap. Replaced them with new Bosch premium parts twice. Used dielectric grease but (({NOT AC/Delco}---Remember this point.))
All new O2 sensors. Two new sets of plugs, first set was Champion, second set was (({AC/Delco Platnums}-correct plugs)). AC/Delco fuel pump, AC/Delco replacement injectors. Felpro (aluminum) lower intake gaskets and top plenium seals. New temp sensor and pigtail for the temp sensor plug wire.
Good Used Parts---
At least three or more times I switched out theses items. MAF, MAP, IAC, TPC, Crank sensor, Distributor, changed Crank sensor and new drive gear-(I added a shim to take up play in the distributor), Coil and Spark Control Module. I know there is more just can't think of it now.
I had access to good used parts through a machanic friend of mine, and yes, ((he made a statement about the Bosh wires a long time ago)). Precautionary tasks---
I traced numerous wires for any exposed or faulty wires. I found two ground wires that were left unconnected during the engine swap, and relocated the ground wire at the back of frame for the fuel pump and sending unit, hoping to fix my irradic gas gauge too. No change in performance or the gauge.
Cleaned all weather pack plugs and used dielectric grease on everything.
Adding it up---
If you did the math I've spent somewhere close to $600 and about a year of any free time surching for the answere. Almost sent an engine to the shop for a rebuild thinking it was an internal valve problem, but ruled that out with a vacum and compression test, doing a compression check with the motor running is incorrect-I saw that mentioned in another post.
Now to the fix--finally--
Two weeks ago I was standing and looking down on the engine bay in complete disgust. Blazer was running like crap. Taking notice of the Bosch wire set I thought could it be or should I just start ripping everything and go old school with a HEI distributor and run a carb.
I jumped in my pick-up(98 GMC w/5.7 vortec), and ran up to a parts store that carries AC/Delco parts.
An AC/Delco cap, rotor button, and wire set cost me another $82, just a $20 more than the Bosch crap, trust me this was going to be my last stab at this fuel injected, computer controlled mess.
I first positioned the motor to TDC.
I ripped out the wires, plugs, and complete distributor.
Verified the AC/Delco Platnums to a .060 gap.
I disassembled the complete distributor and put it back to the stock shim. (I did switch out the Crank sensor one more time with one from a raplacement motor for my 98 5.7 GMC). V6 and V8 motors of this erra use many of the same parts and sensors. A little need to know info for you.
Installed everything taking the time to make sure it was done right. Timing marks dead on, all wires ran correctly and used seperators to make sure none of them touched or ran near anything metal. Oh yah, dielectric grease. Cleaned up and jumped in the Blazer, hit the key and had drove it for about a week without any missing. Did however have a lot of ping while climbing up-grades on the high-way. Caught the post on here about doing a Crank sensor relearn so drove over to my machanic friend and relearned the sensor.
All is good and this time I know I finally fixed my P300.
Moral--
spend a little $$ the first time on quality manufactures parts and save in the long run.
I don't think it was anything to do with the changes I made on the distributor, I had done that many times before. I feel it was the switch to AC/Delco parts on my ignition that did the trick.
I don't feel to bad about the money spent on many of the parts because I plan on keeping this Blazer for a while and I know it's done right. Yet it was a bit frustrating it has been a great learning process.
Good luck to all that are chasing this gemlin and thanks to all of you on here with the great and helpfull tips.
Now I can try to fix the Gauge and the possessed ABS light.

 

Here's to your perseverance!

Do you have a title name or link to that compression post?

 

I couldn't begin to remember the title name. I do know that the person was talking about doing a compression check on different cylinders with the engine running. He stated that the compression was jumping from near (60 Lbs. or so) to nothing at all. Not knowing any better I hooked up my gauage on cyl. 1 and had a helper start the engine. Yep my gauge only read 90 Lbs. at the most and fell from there most of the time, I now relate it to cam overlap. I was talking with my engine builder and later got a second confermation while talking with a friend from work, (also a big grease monkey). He explained the compression test. I tried his method and came up with great compression that was the same on each cylinder. I also hooked up a vacum gauge and was pulling great vacum. Then a tip on unplugging the MAP sensor while it was running and missing, sure enough as soon as you unpluged it the miss went away, I switched out the MAP a few times and for some reason it seemed to come right back. The earlier tests and the tip was enough to rule out burnt or sticking valves so I called the engine builder and put the hold on the rebuild. I sat on things for a few days and when I built up the steam to try again I started from the begining and the first thing that caught my eye was the Bosch wires. You know the rest from there.
Here's a second comformation for you. Another guy I work with was also chasing the evil gremlin on his 97 4x4 S-10 truck, which he had replaced the wires with after markets with no improvements. He'd hearded I was dealing with the same issue and got with me late in that week before I fixed mine. The following Monday I gave him the news and he cracked open the wallet for AC/Delco parts and his is fixed also.
I would have never thought that two brand new sets of wires would act the same way so I had overlooked it. I guess you get what you pay for, and it wouldn't have cost that much more in the begining anyhow!!!
I'm just super relieved that I don't have a bad computer or wiring, in the future I'm planning to swap in a Vortec 5.7 from a truck like my GMC. I've got the shorty headers, can make the motor mounts, just need to locate a good cowl hood when the price is right. I'v read up and it seems to be just about as close to bolt in and plug it up as you can get.
Anyhow I'm not saying there isn't a tool or method of testing compression while the engine is running. Just that it's not how you do it with a gauge like mine.