What fuel system are you plumbed for return style or returnless? What kind of regulator are you running? I had my hose come loose on pump in my tank and only had 20 psi and motor still ran fine at idle and part throttle. I wonder if one of your injectors are staying open or one of your coils aren't firing. Is the PCM tuned for the injectors you have? I have the returnless setup with the corvette filter/regulator and I have 58 psi all the time.