Enginn ís runing op to 3000.rpm in nutral then it gó to normal idle fór about 2-3 mín,then all starts again.
There are several possible causes, including vacuum system leaks, engine coolant temp sensor faults, and a problematic throttle body. What does your shop suggest?
duplicate post for some reason 
Knowing the engine size might help a little, 2.0L 4 cylinder or the 2.7L V6???