Chokes on startup, shudders while driving
 

Hey, I'm a new owner of an '89 F150. It's a four speed manual trans, 4.9L 6cyl. This is my first truck and I bought a beater in hopes of learning while I fix it up. I have minimal experience (biggest repairs have been replacing disk brakes on a previous car and replacing the EGR valve on this truck), but I want to learn more.

The truck has a bypass switch and the forward fuel tank was removed. Both of these modifications occurred before I owed it. Also, the clutch is a bit grumpy. Sometimes I'm unable to get it into first (I usually "Granny start" from second), and every once in a while it doesn't want to go into reverse.

When I bought the truck, it would just choke and cut off after driving for 30 minutes or so. It would restart after cooling down. My mechanic said it was a problem with the fuel pump and replaced it, and that fixed that problem.

After that, the truck began to have trouble starting. I would flip the bypass switch and the engine would rev up, then drop RPM's quickly until choking off within a couple seconds. When I flipped the switch again, it would start up fine. It would run fine after that.

A mechanic recommended having the fuel injectors cleaned. The fixed the problem for about a week, then it started again. A friend of a friend who has a home garage suggested a vacuum leak, and we found a leak in the EGR value, which we replaced.

Replacing the EGR valve fixed the problem again, for a couple days, but it came back. Meanwhile, immediately after the EGR valve was replaced, a new problem started. Periodically while I'm driving (I haven't been able to detect any pattern), the engine will just shudder or choke a bit. It feels like it's threatening to choke, but it never actually does. Once it starts, it typically does this every couple seconds until I shift gears.

I'd really like help in diagnosing what might be causing the choking when it starts and the new shudder when driving. I realize it could be a number of things. I'd like to prioritize things which are least expensive and which would need to be done anyway for routine upkeep. Any ideas?

Thank you!