Ok here's the deal. I've got a 90 Lumina Euro and I've replaced the computer, the idle control valve, and also the crank position sensor. Yesterday I followed the idle re-learn steps, which seemed to help for a little while. The thing that I can't get rid of is this stupid jumping rpm. The car is fine while accelerating but it's when I let off the gas that the rpm will slow with the car, as should, but then all of a sudden the rpm will drop to about 900 and then jump to 2000. It will continue this process until I come to a complete stop. It was also doing this before I did the idle relearn so I decided to see if it was just the computer trying to relearn on its own and I went out and just let the car cruise and let the rpm jump for about an hour on some back country roads and to no avail I got no where. Does anyone have any idea what I am to do? Thanks in advance for any help.

Is the IAC clean and lubricated? IAC air passages clean? EGR functioning properly?

do you hear a loud whissle or sucking noise???
maybe your upperintake manifold isn't seated properly that happend to me once befor l though it was a huge problem turned my New gaskit was bad

So far I have cleaned out the IAC and its passage. I took it out for a drive and everything seems to be going pretty good. I'll keep driving it and go through a tank and see how things are after that. Thanks for all the help.

Any time a component is replaced regarding " driveability & emmisions" or if the battery is un-hooked.....a simple cleaning of the throttle body plate and the bore where it rest's will get rid of all the idle problems.......I can't stress this enough....there is a predetermined amount of air that must pass over the throttle plate at idle....a dirty one will make it surge after replacing a key component or unhooking the battery. I don't bother cleaning IAC's any more.
Phillip