1993 Acura Integra Idles weird when in drive

I had a new motor recently put into my car, and everything ran fine until about a week after. now when i put my car into drive it idle high then low then high and repeats. At first it didn't do it a lot. now it does it every time i am in drive or even in reverse. How do i fix it and is it going to cost alot of money?
I have K&N air intake.