1988 f150 stumped

ok I have a 1988 f150 I've had it for about two
years and it's been doing the same thing only
works during the summer it cranks up and runs
fine but after about 20 minutes it will cut off and
he will here the pumps are muffled 20 minutes
later you will hear the pumps sound like they are
supposed to and then it will crank up as if nothing
ever happened runs fine on the interstate does it
mostly when I sitting idle for too long I have
replaced the bull fuel pump high pressure and low
pressure new fuel pressure regulator new
distributor new thermostat and thermostat sensor
I have also straight wired the high pressure pump
and hook it to a toggle switch today still no change
I'm thinking of doing the same thing to the low
pressure pump tomorrow but I have so much time
and money into the truck I figure why not see if
somebody knows something I'm not thinking of
any help is appreciated thanks