ok my car has a gsr head w b16 block
my car just started to do this when i took off my stock b16 head which was runnin kewl, just wanted to put on my gsr head since i pay lots of money to have it rebuilt. change out iav valve, low coolant valve under the throtle body and idle would be kewl but then it would jump from 900 - 400 or even try to die but it wont ever die. only when i'm at a complete stop, it runs good but that only problem can ne1 help thanks

 

I would check for a vac leak, air in your coolant, and the IACV, in that order.