Loss of power when warm

1998 5.3L GMC Sierra Starts and idles fine, then intermittently when warm it slowly losses power. When on highway at 80mph it slows to 60 then to 40 then quits. On immediate restart it will run for a mile then repeats problem. Problem does not occur when truck is cold. Replaced fuel filter and pump, Fuel pressure is 55PSI. No engine trouble codes. My sons are stuck on the other side of the country can anyone help?

As it turned out the Fuel pressure drops to 47 PSI when warm and throttle is open (High Manifold Pressure) Suspect the fuel regulator on return side of rail.

another thing to check would be the coolant temp sensor. on my 94 5.7 the sensor was bad so it would run great cold but when warm it would die out and stall