Hey, so i just recently started my 86 Lx and after a few minutes the rpm started going up and down randomly. I thought it was because of the first start in about 2-3 months. What do you guys think? Is it possible?

 

Throttle position sensor? My 1st thought. You may be able to find out at your local parts store, by borrowing their tester(if they have the OBDII and EEC IV unit available). If you have any friends with a 5.0, from 86-93(manual trans), you may borrow theirs for a moment and that could answer the question. MotorCraft short # should be CX1133.

 

5.0 or 2.3?

If 5.0:
I would start with the Idle Air Control motor attached on the right side of the throttle body. With the car off, unplug it and then start the car. If the idle is more stable then the module needs cleaning/replacing. I'd also look for vacuum leaks. Hook up a vacuum gauge and make sure you have around ~14hg of vacuum at idle.

If 2.3:
I have no clue

 

IAC was my first guess as well or possibly water in the gas.