Just recently picked up a 1987 GMC R1500 350 with TBI, I originally got the truck and it ran perfectly. Drove it an hour home. Few days later it started acting funny. Started idling funny, stumbling at a stop in gear. Almost like it was gonna fall on its face and than it’d pick itself up. It even died a couple times putting in reverse backing out of a spot if I didn’t give gas right away. Tried checking for vacuum leaks. Couldn’t find any. Replaced/rebuilt all new seals on tbi, even new injectors. Chamged vacuum modulator on the trans. The IAC and the TPS. but now it’s thudding going down the road and won’t even get up to speed.does anyone have any suggestions? I was thinking fuel pump?