Thanks for taking a second to read this.

I have an 89 Cr-x Si that is on its third engine in 8 years (all blown by my own hand). I am having a very annoying idle issue. once it has warmed up the RPM's settle into a nice 900-1000 rpm idle. If I rev the engine even slightly, the idle crashes to around 500 rpms then jumps back up to normal. It does this while driving. If I hit the clutch and let the engine settle, it drops down to around 500 rpm. It shakes hard when it does it. I can give all the stats and recent changes to the car if need be.
My mechanic and I are at a total loss for a solution. Any help would be appreciated.
What controls the engine as it settles into idle?

On a side note. The bouncing Idle thing 1000-1500. I cured it by clearing out my trouble codes from the ecu.

We had drawn that same conclusion. I believe that I killed the IACV that I had. It had been through a couple motor swaps and didn't have coolant going to it for a while. I am going to try to install it tonight, but its raining. That might not stop me.

My Crx is almost completely done and resurrected I've had it for 9 years and as of right now, its running the best it ever has. DON'T GIVE UP YOUR CRX'S!!! They are slowly becoming impossible to find. If your CR-X is nice, Hold on to it.

I will post back after I install it. Hopefully if someone ever Google's Idle Crash they will find this.

i am not trying to be your mechanic but there is a iacv or idle air control valve, if defective then the idle could do just about anything until the ecu controls it by reading the o2 sensor and cps or cylinder position sensor. if i were you i would definitly try to check that valve out it has wires pluged in to it and will be on the intake plentum.