Volvo Repair: 91 volvo 740 intermitently loss of power as if no fuel is going to the motor., volvo 740, intermitently

Question
Hello,
 I have a 1991 740 volvo. The problem has been that sometimes when i drive the car, i will give it gas and the car starts to slow down as no gas is going to the engine.
 We replaced the fuel relay and the fuel pump. The volvo ran fine for about two tanks of gas and now it is back to the same problem. I will be driving just fine and then i start to feel deceleration. When I press on the gas peddle  there is no acceleration. I have to pull over and sit for about 5 minutes and then restart the car. It always starts back and if I am lucky, I make it home. Sometimes it will only drive one more mile and the cycle starts over.
 
Do you have any idea what else could be causing this?

Thanks so much

Answer
  Yes, without ever touching or seeing the car. Which fuel pump did you repalce? I'm going to guess the main?(Big one) If so, it's probably the the in tank pre-pump that's at fault. Another common fault is the pickup hose on the pre-pump is deterioted or gone. (rotted off)
  What happens is a good strong main pump can suck hard enough to bypass the pre-pump and lets the vehicle run until it works so hard and gets so hot that it starts percolatong the fuel at the fuel pump head and trys pumping foam or is so weak it can no longer maintain proper pressure. Cool it down it works again for a little while. Sound like what's happening to you? I think so.

Mark