i just put a zc in my crx and when it is at idle it acts like it is going to die the it revs back and it does this over and over, its a 90 fuel injection and its not a vacum leak that i can find; i think it might be the atmosphere persure sensor because the zc doesnt have one that i could find; it does ok while the choke is in but once it heats up it does that idleing thing again

 

Try a different fast idle control valve (located on the back of the intake manifold).
This is a common problem on used engines being swapped in.

 

Also, it could be something super simple like the idle screw not being adjusted out enough.

 

It sounds like it's running to lean, I think.