broke91firebird

Last year my 1991 firebird with a 31. and 5 speed had an injector hang open 2 hours away from home. The result was a bad injector, cracked right exhaust manifold, melted Cat and a melted EGR. I replaced the EGR and whole fuel rail(two other injectors were borderline shot) with good used parts and the cat with a Cherry Bomb muffler, which sounds great with no muffler .
Now I have been driving it for two weeks no problem other than the tack starts jumping 300 RPM at a time over 2500 instead of a smooth climb like it always was. On my way up to Michigan for a family reunion I got a check engine light idling around town. Figured it was a result of the modified exhaust and hole in the manifold. Half way up I was passing a guy on cruise when suddenly the engine flat-lines, comes back to roughly 500 rpm and dies completely.
After that it will start, idle super rough then die. Changed the fuel filter and ignition coil(out of a 4.3) after which it seemed to run great. Yesterday the tach started ratcheting up about 300 RPM again. I spun the tires a little in town, maybe getting up to 3000 RPM and when I let off it, the RPMs dropped all the way to zero AGAIN. after priming the pump a few times(key on 5 sec off 3 sec), it ran but super rough.
Towed home and now seems to run fine again.

Anyone have any ideas? The hole in the manifold is directly under the coil btw