I have A 94 Z28 when i start it up it runs fine, at about 3000 rpms the tack needle jumps and then it backfires 1 time and engine lite comes on it will run good for a few miniutes and then start crappin out. I put a new dist cap and ign. mod. a new coil.the comp. will not put out any codes.so i replaced that too. it still does same thing fuel pump is fine.can anybody help very confuesed