I have a 3000gt sl that runs fine but all the sudden won't idle. Any ideas what the problem is.

 

Hmmm, I remember when I had my 3000gt base model, and would come to a stop after getting off of a freeway exit, it would have idling issues and would even shut off sometimes.
now, this was several years ago, but I think the fix was replacing/cleaning the idle air control valve or perhaps the throttle body itself. I think the "pintle" on the IAC can get dirty and get stuck open/closed and restrict air flow at idle. Good luck!