1990 Pontiac Firebird No Start / weak injector fire

Engine Performance problem
1990 Pontiac Firebird V8 Two Wheel Drive Automatic

The car will start somtimes but when it dose it will run for 20 to 30 min then the injector volatge will drop out and the noid will show a real weak light. Change ECM, ignition module, the 2 things that control the injectors. Changeing of the ECM with a used one and an remanufactred ECM cause a no injector fire at all. Change the EPROM from the old ECM to the new and same thing. Any Ideas what could be the problem.

Thank You