1987 GMC S15 truck just goes to "idle"

I have an 87 s15 w/2.5l I4. What happens is completely irratic. I can be driving and all of a sudden the truck will go back to idle, i can let off the gas and push it again, it will try to rev back up then same thing, back to idle. I it does it when holding the pedal steady at different speeds, conditions, road terrian, ect. All i have to do is pull over, shut it off and restart and its usually good for another few minutes. I replaced the IAC valve because it kept saying "rpm + or - 50rpm from were it should be" , The fuel filter, the O2 sensor, none of which helped. I dont know if maybe its the fuel pump, regulator, injector but it seems to be with the fuel system not the engine or trans, unless its a sensor. I have no idea but its made my truck a hastle to drive... help me please