non starting
 

1989 f150 302 eng automatic. month go started dying after switching to front tank after about two minutes. replaced fuel pump inside tank(needed. new guate anyway) replaced high pressure pump.drove
well for about one week. and then would drive about two miles and when switching to front tank would die.replaced dual function reservoir and drove fine for two weeks. this week end after driving about 5 miles engine started running rough during acceleration acting like spark plug wire off. eventually truck stopped running, would let sit for about 25 minutes and could drive another mile. Got home, checked fuel pressure, was 35+ lbs for about 5 minutes while idling, would drop to 28, drop to 20 and engine die. happened a couple of times and then eventually just the 7 lbs pressure and no start. exchanged high pressure fuel pump and put on new fuel filter just in case. no luck, pressure no more than 7lbs. clamped low pressure return line and no change in fuel pressure. checked codes with scanner and shows 121 and 122, TPS. Am I wrong in thinkg TPS would not effect fuel pressure? any help with problems greatly appreciated.