okay here is the deal its a 95 ls that has had lots of stuff done to it in the past from the shop that owned it then converted back to stock. My sister bought it and has been building a new motor for it but the stock motor that is in it right now starts idling at 2000rpm then it will go down to about 800rpm then back up to 2K. It sounds like somebody is stepping on the throttle and releasing it. It does this all the time and when it is warm or cold and nothing we check or do seems to work. No CEL, No codes. Any ideas?

 

Rebleed your coolant if you happen to play with it recently, if you hadn't check your vaccum lines may have a leak, clean your IACV with gas or choke/carb cleaner