1994 SierraGT Engine Performance Issue

I have a 1994 GMC Sierra GT P/U with a 350 and A/T. About 2 months ago, I discovered that when turning on the A/C, lights, turn signal and/or any other electrical load, the engine would begin to miss/backfire. In fact, sometimes the backfires were excessive (like a shot gun). Removing the loads would improve performance but an intermittent engine mis-fire remained. Sometimes, this condition would persist. At that point, I found that turning the engine off while the truck is still rolling and then starting it back up again would eliminate the mis-fire. I believe this is due to the fact that the ECM is in open loop mode. However, this symptom will always return, especially when the lights, A/C, and radio are turned on.

I have taken the truck to several mechanics and both have told me that there is an issue with the oxygen sensor. For some reason, the oxygen sensor starts out functioning properly and then the signal goes low/flat. At that point, the ECM takes that to mean a lean exhaust and subsequently increases the pulse count of the injectors. This causes the fuel mixture to become rich. I believe the "shot gun" explosions are due to the ignition of the excessive fuel in the exhaust pipe during engine mis-fires. The last mechanic basically installed a shield around the oxygen sensor. The signal integrity improved but the problem was not solved.

Before I took the truck to either mechanic, I had already replaced the following:

oxygen sensor, coolant sensor, alternator, EGR valve and EGR valve switch, MAP sensor, coil, plugs, plug wires, cap, rotor, ignition module and pickup module.

The one mechanic did try another ECM but used my original MEMCAL in it. The problem still existed. I have only received trouble codes related to EGR, lean exhaust and MAP sensor. I have been told that MEMCALs can fail and the ECM will not necessarily set a trouble code.

Could the symptoms I have described be related to a faulty MEMCAL? ANY thoughts on the matter would be greatly appreciated.

Thanks.

Sounds like a bad alternator to me,i would start by re-checking the charging system,even if the battery light is not on does not mean the alternator is putting enough out, even new parts can be bad. Also make sure all ground and power wires are clean and tight, especially the back of heads to firewall [if that model year has them]