Idle unstable when AC is on

[gepeto]

On my KA24DE powered 240sx (95) when I put the AC, the idle jumps to 900-1000rpm - but never stabilizes. It goes down to 500 2seconds later, 1000, 500, 1000, 500. I don't remember if it was doing this last year (I use my AC rarely and even more rarely when idling..)

I noticed it yesterday as I was outside of my car and heard it go VRR....vrrr...VRRR...vrr...VRR...

Any idea what could be wrong?

Note: The AC works well, my idle is super fine when the AC is off, and my check engine light has been on for a little while.