well, dont know about the fuel pressure, but i had the exact same problem with my 89 IROC. i did a half assed induction service, BG induction cleaner sprayed into the throttle body, cleaning up the IAC valve, and wiping it out with a rag, cleaned out all kinds of black crap, probly resens and carbon build up. but its been running like a champ ever sense. as far as the fuel pressure dropping, sounds like it could be a leak, more along the lines that the fuel pump actually turns off. see if you can get to the connector to the fuel pump, if not the one on the sender itself, then the connector farther up, not sure if its the PCM or what that controls it. but put a test light on it, and watch the light, if it turns on, and then goes out, your loosing the signal to the pump, and there is probly something wrong with your PCM. but id have to go schematic searching to find out.