Car stalling AGAIN, Updates- none of them good. Options

[rjames]

Drove around for 30 minutes in 80+ degree weather. All arterial roads, so several stop lights, etc. No problems until I was almost home. Car stalled when I stopped at a light. RPMs just dropped to zero. Was able to start it again with a little difficulty and had to keep my foot on the gas to keep it from stalling out again. Power seemed ok though the rest of the way home (I think). It stalled once again at a stop light a couple of blocks from home and then again when I pulled in the driveway. I let it sit for 30 seconds, started it again and it idled just fine. Rock solid at 1000 RPMs. Pulled it into the garage where it sits now.

Prior to today the car has been running and idling perfectly since I went through the FI parts last year, which included installing a 123 distributor, new CHT sensor, a newly rebuilt mps (calibrated for correct afr on my engine), new vacuum hoses, NOS throttle body, rebuilt injectors, new fuel pump, valve adjustment and timing.

I haven’t checked the AFR since last year, not sure why it would change though.
It’s a ‘75 so the fuel pump is in the front, so I can rule out vapor lock I think.

Anyone have ideas as to why it would be stalling?

[rjames]

Went to rebuild the MPS.
I opened it up and the diaphragm was fine. Cleaned everything off put it back together and it still didn’t hold vacuum as well as it should. I actually have another MPS that I tried to remove the outer screw on awhile back and totally buggered it up. So just for fun I took the bottom part (where the vac line connects to) of the MPS I was running in the car and replaced it with the bottom of the other MPS I had and cleaned all the seals gasket etc. and put it back together to see if it would hold back in better.
Vacuum held better, a little bit of a leak down but less than 1 inch after five minutes. Installed it an the car stayed running. Got the idle dialed in. Great, fixed!
Went in the house for lunch and came back out an hour later to drive it.
Car wouldn’t stay running again,
Tried swapping the coil with a known good coil- no change.

Then I realized I didn’t hear the fuel pump running. Discovered my relay board was bad. Man, all this work and it’s the fuel pump. How did I miss hearing that it wasn’t running?! Swapped out the bad board for a good one and the pump starts running again and everything was great car idles perfectly. Took it out for a spin and as soon as I drive 1 block and let off the gas the car stalled again. I verified that the pump is running so the bad relay board must’ve just been a fluke in the middle of things.

I’m at a complete loss now. MPS is leaking but just barely. Is it still a suspect?