Cadillac Repair: i am working on a 93 deville..., cable location, black paint

Question
i am working on a '93 deville with a 4.9 engine. engine has an ocassional misfire. when you turn on the ac it stumbles badly. scanning it with my mt 2500, when the misfire occurs, the timing gets radical, that is it jumps from anywhere from 0 to 40 and anywhere in between instantaneously. i changed the est module, with no success.(no codes present) i changed the wires and plugs, wires where leaking, didn't help. checked voltage, a little low (13.7 without ac on, drops to 13.1 when on). hooked up shop battery charger, noticed great improvement some evidence of possible problems with the rectifier in the alternator. changed alternator, picked up .5 volts. didn't help, same condition.  ran jumper to distributor, thinking maybe not getting enough amperage, no change. arced jumper to ground, condition improved greatly. pcm was swapped out, condition stayed the same. any guesses? let me know, thanks. my phone # is 5044317806 any help would be a godsend

Answer
Hi William, It was good talking to you on the phone. Afetr rereading your email I still feel that cleaning the nregative battery cable end, starter bracket or all black paint at the cable location and cleaning the block area where the cable touches should solve your problems. Let me know if you had success or if I can try to give some other suggestions. Bill