Thanks for all the useful info.
I drove about 30 miles and made a few stops today, didn't act up at all.

The last time it dropped pressure, I went into the auto parts store for about 5 minutes.

I came back out, PSI was still 6. I started the engine, it ran about 20 seconds while I put on my seat belt, and dropped slowly to 0 PSI.

I didn't even back out, as I thought the car was going to stall.

I revved it up to a fast idle, around 1500 rpms, then after another 10 seconds,it came back up pretty fast.

I bought it from Rock Auto, and they quit carrying them in the last month.

I can send it back for credit, or pop it back apart and see i there might be a valve problem. The cost for a rebuild kit from Mancini is more than I paid for the pump.

I just can't leave it this way. I just put the gauge on when I installed the pump, so I don't know how the last Muscle car pump acted, but no fuel pressure just shouldn't happen.

I ordered a Comp Cams 4626 fuel pump rod to pop in while I have it apart.

Thanks again, I'm baffled.