[Walt Fricke]

A lot of racers connect the rears of the two fuel rails together with a length of hose. The theory is that if some obstruction shows up somewhere in the loop, having access to the float bowls from either end should keep it from causing mischief. Although I am dubious that these straight forward and relatively large passages could have issues like this, I do it on my engines.

With that fuel pressure gauge you ought easily enough to see if there is a pressure difference between starting up fairly cold, and warm and hot running. I suppose you could have proper pressure at idle, but if flow is inadequate from the pump while hot, at WOT the pressure would be lower?

If you have any kind of data logging, it is easy enough to buy a pressure transducer to send a variable 0-5VDC voltage to the data system, where you can convert that to PSI. Then you can record and compare what the 1st run does vs. the final, rough running, runs.

Is your vacuum accumulator connected to the fuel pressure regulator, or is that just a choice of where to mount things?

The vacuum line goes to your distributor? Is that an idle retard? If the distributor centrifugal advance somehow gets sticky when hot, could that cause these symptoms? You might compare how it feels to your fingers when you pull the cap and advance the rotor against its springs when cold, and when hot.