NA fuel system problems

[Sleeper-6]

I put this in the lounge since it's not a buick.
Anyway a friend and I have been working on a 66 Mustang coupe with a 10:1 302, Roush heads 228 cam and a bunch of other goodies. The only thing that's keeping it off the road is a strange problem with the fuel system.
The car has a stock tank, feding an external canister filter and a Summit billit fuel pump plumbed with -6 Braided line to a Summit adjustable fuel pressure regulator to an Edelrock carb. The problem is that after about 5 min the fuel pressure starts slowly falling off until the car will no longer keep running.
The pump is the second one and is brand new, If I crack the fitting in the inlet side of the pump fuel pours out, so I know it's not starving for fuel.
Has anyone seen FPR fail and cause this? Where else should I begin looking?

 

[TurboDave]

Is the tank vented properly? If it's sealed too tightly, then....

 

[Sleeper-6]

The cap is new and vented, It will do it even if the cap is off the tank.

 

[Sleeper-6]

We did some more playing around today, The pressure form the pump to the reg does not drop off, only after the regulator. I'm going to swap gauges today to make sure. Prob pull the reg. apart. This is all new stuff, unfortunately we bought it 2 years ago and we are just getting the car back on the road now and de-bugging everything.