I just got an '86 prelude with the 2.0 and every once in awhile, the idel goes up and down. I had this similar problem with my ef and that was the fitv(fast idle thermal valve), but im not sure if this motor even has one of those! Sorry for bein a n00b but ive never worked on a prelude before. Oh and one more thing the pgm-fi light comes on every once and awhile, usually when im pushing it hard. Any ideas?

 

Yes, your Prelude has a fast idle valve. The most common cause of a surging idle in that car is a low coolant level. If the coolant level is good then it's possible the fast idle valve has backed out a little. Remove the cover and screw the valve down a turn and see if that fixes it.
To know what's causing the PGM-FI light to come on you'll need to get the DTC (diagnostic trouble code) from the ECU. The ECU is in the left side pocket of the rear seat. Remove the ashtray and you can see the display light to get the code.