I got myself a little 1991 Isuzu 5 speed, space cab with the 2.6

Surprisingly a lot of things still work, ac/heat, all the lights, doors, tailgate. Starts right up but idles rough and am having this issues where when I shift into 3 or 4th it quits. Seems like it’s starving for gas but I pop it in neutral and hit the gas and it usually runs rough and staggers in gear unless I kill the truck and cycle the keys starts up again no issues. Know very little about this truck. It came with a bunch of parts, some changed some not. I’ve got a few hunches. Starting with fuel filter and pump, but it runs okay after the issues and does seem to only happen after shifts so I’m starting to lean towards vacuum line as the idle is intermittently rough too. Lastly and I sure hope not but could be valves?

Don’t really want to guess and fix everything as I’m on a tight budget for the truck, still needs tires all the way around too. Hoping someone sees this and can give me insight both as to the issue and maybe a good video resource to fix it. Thanks for reading