i had exactly the same problem and it happend all of a sudden while driving, the car had absolutely no guts and was choking then it will drive normal for a little bit.
check that the plus wire isnt earthing. the dude i brought the car off changed the fuel pump to a walbro and he used electric tape instead of heat shrink to cover up the solder welds and it was earthing.
i took the pump out re did the wiring and it was all good after that.
Such a small problem i had to pay $150 to get the car towed home
I would definately check out the wiring as you got an aftermarket pump.