Next step?

What is the fuel pressure?

Don't know if you're still fighting this, but check to see if the fuel pressure regulator is draining the fuel pressure off back into the tank. To do this, you'll need a pair of needle nose vise grips, and if you can obtain a fuel pressure gauge that'll help.

With a helper inside the car while the engine is running, have the vise grips ready to pinch the fuel return hose clamped onto the fuel pressure regulator. Have your assistant turn off the engine. As he does so, clamp the return hose shut. Allow the car to sit the requisite amount of time for the condition to occur and attempt to restart it. Allow the engine to run only very briefly with the vise grips installed, as the fuel pressure will be very high due to the blockage of the return hose. If the condition does not occur, the problem is with the fuel pressure regulator.

If you have a fuel pressure gauge installed, you can simply monitor the fuel pressure. If it drops when the return line is pinched but not when it isn't, the problem is in the fuel pressure regulator.