I have a 1993 civic cx, with a jdm b16A motor. When i start it to warm up it will rev up to about 2500 rpm for 2-3 minutes. (which is normal when its cold) But after that it starts revving up and down until it gets all the way warm. What could be causing this?

 

i had that problem on an accord i had, it was the thermastat...when u start the car
try not to touch the gas at all until it reaches operating temp (the temp gauge get's out of blue)
and the idle shouldn't be so bad...
i just removed the thermastat and my problem was fixed, hopefully that helps u man!
also!! check your coolant level, make sure u got full coolant in the radiator plus overflow
so that your motor isn't starving of coolant!

 

I would start by cleaning your FITV(Fast Idle Thermo Valve) and the IACV(Idle Air Control Valve).

IACV: Located on the back of your Intake Manifold plenum by two 12mm bolts. Has two coolant lines running to it. Take bolts out and clean the metal screen with brake cleaner and an old toothbrush, etc.

FITV: Click link for a walkthrough on how to clean it. This walkthrough shows the intake manifold off, I assume he was doing something else and thats why he choose to take it off. I usually take my throttle body loose and then remove the FITV from the bottom of it on a work bench. Be careful in not tearing the throttle body gasket.

https://honda-tech.com/forums/acura-integra-6/%2A%2A%2A-how-%2A%2A%2A-clean-your-fitv-aka-fast-idle-thermo-valve-1564019/

Good luck, hope this helps.