The running fine, then rough, Jekyll & Hyde thing also happens when one of your coil packs blow. I had one go out recently, replaced it with a cheapo $25 off brand one...it died in less than an hour, tried another and it has been fine since.
IF you want to test the fuel pump, you can open the feed line at the fuel rail, pop the cover off the fuel pump relay (under passenger's feet) and push the points on the relay closed. I wedged them with a piece of fuel hose against the adjacent relay.
Then when you switch the ignition to RUN, the pump will run continuously. Fill a mason jug or the like and helper switching on/off. Strong steady stream. If not, check for power in to the pump through the access cover - passenger's side of trunk floor nearest to back seat.