Most of the "grounds" on these cars are total sh*t from the factory. For me, first step would be to go through every ground in the engine compartment and re-do them. Particularly bad ones are the fenderwell right next to the battery, ICM right underneath the coil bracket, and the ground strap from the starter side of the engine block to the body.
That could fix some of the problems, I'd also check the harness from the injectors going all the way to the PCM. It could easily be as simple as a broken wire somewhere. And it's rare for a PCM to just go bad, usually it's because of a mistake while tuning it (losing power while writing to it and bricking it) or physical damage/water damage. Not unheard of, but highly unlikely.
