1992 Ford F-150 bad fuel mileage, sluggish, stalling

Truck replaced o2 sensor, coolant temp sensor, fuel and air filters. Has 2 fuel tanks, but prior owner disconnected one and shut off lines to it. Vehicle will start and when you drive down the road it will start running sluggish and eventually stall. Sometimes the fuel pump runs constantly with switch in on position and sometimes it buzzes momentarily and operates normally. Any idea where my problem could be.
Thanks,
Rodney