Possible exhaust problem
 

O7 rx8 idles up and down, from cold,crank u would think it was low compression but,if,u drive it for a lil bit it goes away drives great. Its sucking gas. Im thinking stopped up catalytic converter? And ideas. Its pulling p4011 code too.

Idling up and down is usually a sign of a vacuum leak. Definitely check your cat, you don't want to drive on a plugged cat, it will kill your engine. But I feel there is more to it. What work has been done on the car?

The motor was dropped in. Its 45k miles its donevthe idleing thing since the swap.ive changed coils and thats it