idle issue -
 

I just completed an upper end rebuild on my 91 964 C4. Every once in a while when warm and coming to a stop the idle stutters below 1000 and the CEL comes on. Once I accelerate the CEL turns off and it seems to be idling just fine. The car has a cat delete, chip and after market air intake. Any ideas? Thanks for any help you can provide!

Have you checked the ISV valve?

You will need a diagnostic tool for the CEL or you will be shooting in the dark and replacing parts for no reason.
The only thing free is cleaning the idle air control valve and tightening intake clamps and bolts and whatever was touched during the top end rebuild.

Was it doing it before the chip and aftermarket parts?

I have a cat bypass and it's fine.

I finally got the codes "23 oxygen regulation stop" and "32 knock sensor 2"