a minor problem with my friends 97 GST

My friend has a 97 GST hardtop w/ like 4x,xxx miles...and ever since he got it, it's had this problem...when he starts it cold, it goes to around 1500 rpms, but then drops down quickly to like 400rpms, then raises back up again...it keeps bouncing like this untilt he car is warmed up...then it bounces from around 900 to 1500rpms...i was thinking it could be a vacuum leak, but we're not 100% sure....any ideas?

Could be getting too much air by the throttle plate, hitting the decel fuel cut. I'm having a hard time thinking about how that works though for some reason. I would start by verifying that the ISC functions. (Remove it (2 8mm bolts), with it plugged in, turn the key to the on position. It should move in and out smoothly. If it doesnt move, or stutters, its bad) Then I would reset the ECU so it re-homes the ISC (no closed loop feedback on ISC position, it can get out of sync with the ECU. Resetting the ECU will force the ISC to the hard stop, so it now knows where it is again). I would then hold the ISC at the center of its travel (with the scan tool, or by grouding the pin on the 95s, I dont think its htere on 96-up though) and set the BISS properly for warm idle. If it still doesnt behave properly, it might be time to suspect the FIAV (fast idle air valve).

Thats how I would approach it anyway.

Edit> just remembered www.crebotech.com. This link here http://www.dsm.org/how-tos/tttt/tttt3.html at the bottom of the page mentions that decel fuel cut hits at 1500 rpm, sounds like we could be on to something with that. Anyway, tons of info here...