Ok this issue is really confusing. We took it to the dealer and they are lost on it as well. I have some ideas on what it could be but i want to hear thoughts from others that have ran in to a solution for the issue.
Issue:
When I take the car on the freeway (55-75mph) it has a jerking motion. When accelerating up threw the gears it sounds like it is misfiring. But plugs, wires, distributor, dis cap, rotor, ignition coil, computer, fuel pump, fuel filter, timing set have been replaced checked and is good. Also replaced TPS.
It goes in to gear just fine. But it just likes to jerk going from rpm's 1800 to 3800. The dealer did thier tests and said the problem is so hit and miss they can't figure it out.
Attached is a running dig code scan for it. I myself dont understand any of it so it might have the solution in there somewhere. If not any ideas what could be causing it?
Oh there is no dtc's or upcoming dtc's.
engine was rebuilt head to tail. New everything.

 

This sounds similar to what was going on with my 00 4x4 ZR2. I have yet to find the exact issue but have done some stuff that has me on the right path.

What was happening for me was only at highway speeds and cruising along at 70 to 75 for around 20 mins. It was hit or miss but eventually I think its when i hit a bump or two that it happened, but not always.

It then rev's from the 1800/ 2000 mark up to 3500. When that happens I pull over, turn the truck off for all of 30 seconds then start her back up and keep going and she would be fine for a while longer.

As you could imagine this was a HUGE pain. I ended up doing a fluid flush and dropped the pan completely. I didnt have any shavings which was a relief but It turns out the tranny filter was very loose and the ring that holds it in place was no longer there. My buddy (A mechanic) thinks that the filter would get sucked up when turning the car on and when hitting a bump would fall back down and it would rev high. Turning it off then back on would allow it to get sucked back up. Seemed good in theory.

We ended up replacing that and the ring that holds it and in the last year or so its only happened once since then instead of the almost daily that it was.

So you may want to check there first.

Edit: After replacing the filter and ring I went on two vacations over the span of a year. One was 3 hours away and the other an hour away. The one from 3 hours away didnt have any issues with it. The other one I was towing a small trailer and hauling a heavy canoe on top and it happened. I pulled over, cycled the engine and was fine. On the way home though it didnt happen. So like i said, hit or miss.

 

i flushed trany 3,500 miles ago replaced filter and new fluid. also replaced vacuum lines. A slight carbon build up is accoring on the distributor cap. My guesses were i still have a vacuum leak somewhere. or my timing on the distributor is off one tooth. My other though was because i had to go .003 on the boor of all 6 pistons, i might need a high performance ignition system but i don't want to pay the price if it is not needed.

 

Jerks/ misfires accelerating.......lack of fuel pressure for acceleration?

I'd test the fuel pressure.

 

That's one thing i haven't tested while engine is warm but when fuel pump is activated, you can hear it. And we tested the pressure then and when starting the blazer up. It is correct but you have a point i want thinking about. The fuel pressure could be lacking at higher revs. Meaning the pump could be failing. I'll check in to a rebuild kit and fix that. But it still leaves me with the same question. Could that be causing the carbon buildup on the distributor cap and rotor?

 

If the distributor is off one or more teeth, the SES light will come on and set DTC P1345. If the SES light works on the bulb check, and it stays off while the engine is running, the distributor is meshed correctly with the camshaft. Along that same line, did you check and adjust camshaft retard? If/when the distributor is removed, it needs to be checked/adjusted with a capable scan tool. Spec is zero degrees plus or minus 2 degrees. Adjusted by rotating the distributor. If cam retard is not correct, you can expect performance issues with no SES light or DTC's in memory. The farther it's off, the more issues you'll have. If it's off ~27 degrees or more, the SES light will come on. Incorrect cam retard will affect how the engine utilizes the amount of ignition advance that the PCM applies. For example: If cam retard is off by 20 degrees, your ignition timing is off by...... 20 degrees, and NO SES light!

If the timing cover and /or crankshaft position sensor are moved, removed, replaced or disturbed in any way, the crankshaft position sensor relearn must be done with a capable scan tool.

Carbon in the distributor cap: When the distributor cap is replaced, a small amount of silicone dielectric grease is applied to each terminal inside the cap. When the engine is running, the voltage passing from the rotor to the cap cause the grease to crystalize on the terminals.... normal, supposed to be there.

Fuel pressure: It is not monitored by the PCM, so if there's a problem, the SES light might not come on. Key ON, engine OFF, fuel pump running, fuel pressure must be 60psi to 66psi and must remain above 55psi for at least 10 minutes after the pump shuts off.