I just posted this in another thread about the same issue. Are you driving an auto or manual? I was having this issue with my 91 automatic after my EGR valve (only on automatics) wasn’t holding a vacuum. It seems to happen whenever air isn’t flowing properly into the engine, be it vacuum leaks, gunked up throttle bodies, etc. If you’re driving an auto, this probably happens when you put it in park right? One thing to also check is your throttle cable. If it’s too tight such that the leaf is a bit open, that can also throw off the idle speed and so the computer will basically be “searching” for the right idle speed. I think some forum posts often refer to this issue also as “hunting idle” or “roaming idle.”
Here’s some other links and videos from my own research on the issue back when I was fixing it:
Surging/Hunting Idle when Parked
https://www.team-integra.net/threads/iacv-fitv-idle-problems-and-timing-adj.95330/
https://www.team-integra.net/threads/how-to-clean-your-fitv.105367/
https://www.team-integra.net/threads/how-to-clean-your-iacv.106451/