PuddleJumper

Hello all - My name's Alex, and I'm new to the board.

My sister has a 1984 4-door Cherokee sport, 2.5L/automatic, 141,000 miles. The engine was rebuilt and installed in March, and it has been driven regularly since. It hadn't been taken great care of before we got it, and had sat outside for about 3 years.
After putting the engine back in and getting it back on the road, it ran pretty well but never perfect - it was always kinda quirky, but ran plenty well enough to get around.

Several weeks ago, all of a sudden, it didn't want to start. It cranked very slow, and would barely start (if at all) when warm. After countless hours of troubleshooting and fiddling - and several "smoke and spark" moments - it cranked fast once again, although the tranny park/neutral safety switch had gone bad so an auxilary ground to the solenoid was in order. (Also, during that time of working on it, the master cylinder and spark plugs were also replaced, for unrelated reasons.)

NOW, a whole new problem has arisen - it's kinda sporadic, but it starts halfway easily, runs pretty well until it warms up, then eventually starts sputtering/missing and dies. It tries to restart after that, but will only run very roughly for a few seconds at near full throttle before it dies again.

With all the vacuum and electrical on this thing, I'm stumped - any ideas?

Thanks,
Alex