12 is good. 8 is on the low side. Could be a pump or maybe a small hole in the rubber hose on the fuel pump (in tank). When at 12 psi, is it running properly?
I had a similar situation once. Turned out to be a bad coolant temp sensor (on T-stat housing). It was sending a signal that the coolant was like -45F. The computer was dumping fuel in it to try to compensate. This would drop my fuel pressure a little. Eventually, it would die at idle when I stopped at red lights and stuff and sometimes wouldn't restart. This might be a less common problem though. Next time yours is doing it, unplug the temp sensor and see if it improves.
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