1995 surburban will not communicate with scanner or run

1995 chevy surburban 2500 4x4 454 130000 miles.
Truck slowly got worse, totally gutless. Replaced fuel pump, cap rotor, plugs wires, air cleaner, nothing changed. Took to shop, the timming was way off, adjusted timming, drove out of shop, ran awesome for 1 and a half days. On day 2 went to start, and the whole truck bucks and a hard start up, engine turns over a long time then it fires and bucks like heck. Took back to shop, "timming out again, "will not keep time. Shop scanner will not commuicate with truck. Replaced computer with after market comps (2), still will not commuicate with scanner. On fourth computer still wont commuicate. Checked all wires from lbd1 to comp and dist. Check dist. Modual all checked ok. Checked all fuses with test light, all ok. Truck will start with timming wire unplugged, idles perfect, at 1500rms it chokes and dies. When timming wire plugged in, truck will not start. Tried 2 different prom cards in 4 different computers. 1 original, one from pick and pull from exact same surburban. Shop and myself are at wits end, any help would be great.