Thanks for the reply!
Can they fail without setting a code? I think I have on old analog meter around somewhere I could use to check it out. I'll put it on the list for tomorrow morning.

 

Took the car out for a test drive this morning after checking the TPS and the relative temperature (as best I could) of the exhaust manifolds with an infrared thermometer. I used a digital multi-meter and checked resistance. As best I could see (?) with a digital meter, ohms appeared to increase nicely as the throttle was opened. And the temperature of the exhaust at idle on all but #3 (friggin' heat shield!) were pretty much the same.

In town, at part open throttle, maybe up to 25% open, it was missing and jerking pretty badly. When I got it out of town on some open road and opened the throttle about half - wow! it pulled like crazy, shifted and kept on pulllin' without missing a beat. How curious. Only misses at small throttle opening. Does the same on cruise at 70 mph, but the miss is not nearly as noticeable as when starting out from a stop. So, load and throttle related? By the end of the ride, it seemed to be doing a lot better in town, but the light acceleration miss is still there. Temp sender for gauge or gauge is bad maybe...temp never showed more than 110F even though the top radiator hose was hot, and heater was blowing good heat.

I had read the 8-page thread on the dread P0300 and had checked the ICM mount for heat transfer paste before the drive. But, I have no CEL and no codes set. I will go looking for my analog meter to re-test the TPS. After that - MAP sensor and EGR.

 

Checked the MAP sensor: 9.8 volts @ atmospheric pressure, 0" Hg; 3.4 volts @15" Hg; 1.7 volts @ 20" Hg. Removed and cleaned EGR valve - wasn't too dirty, gasket was good. Double checked TPS with analog meter. No flat spot, smooth increase in resistance checked repeatedly in low throttle opening area. Checked for codes. Heh-heh, after the test ride it threw P0300. So at least I don't feel so lonely now. Now am considering new plugs wires, rotor and cap.

 

Woo-hoo! Googled "4.3 vortec p0300 light acceleration" and found no shortage of advice for this common problem. One "expert" said the problem was valves sticking in their guides and both heads would have to be done. Others had other solutions, but this thread: http://www.s10forum.com/forum/f107/4...dition-322478/

seemed to describe my symptoms perfectly and the consensus fix was to replace the distributor gear (if it was worn). They even provided a GM part no. for a kit to replace the gear.

I pulled the distributor and sure enough, my gear showed more wear than the the one pictured in the thread linked above. I was going to get the kit when I noticed that the threaded holes for the cap hold-down screws in the plastic base plate had broken on one side and cracked on the other. I opted for a rebuilt distributor from O'Reilly's for $90. While I was there, I got a new cap and rotor. Thirty minutes later, my miss was gone. The solution to this (unless I missed it) was not mentioned anywhere in my AllDataDIY diagnostics.

Summary: New ECM............ $115 (unnecessary)
Flash fee...............$75 (unnecessary)
Core.....................$75 (unnecessary)
Ignition switch........$80 needed to restore spark
NGK plugs..............$13 needed to restore spark
Distributor..............$90 needed to restore timing
Distributor cap........$33 improved ignition performance
Rotor......................$8 improved ignition performance
Total spent............................$489
Cost for needed items..............$224

Well, I could return the new computer and reduce my total outlay to $419, but I think having a new-flashed ECM for the truck is not a bad thing.

Thanks to all who helped and to all the guys who maintain forums like these. I cannot imagine what we would do without them. I hope this helps the next guy with this problem.

 

Outstanding Bill! Glad you got it figured out. Too bad you spent a lot trying to figure it out, well at least you replaced a lot of questionable parts that will not need to be replaced anytime soon.

I recommend a sticky!