Engine idle too fast thenslowly

Sounds like a throttle position sensor

I had the exact same problem.\ and it had me dumb founded too. After checking electrical and fuel things and codes to no avail, I went the throttle position sensor (TPS) route but no such luck either. Then I realized it could be the air bypass valve (APV) which is what is used to set the high idle when the car is cold without having to actually actuate the throttle plate.

The APV is a little electromechanical (selonoid) valve (much like the canister purge valve) that lets addtional air into the manifold though the throttle body past the trottle plate (why it's called bypass) to increase the idle RPM. Its that 3' long black cylindrical thingy connected to a wring harness and attached to the trottle body right near the TPS. I pulled mine and it was all gummed up with carbon and was unable to actuate freely (hang open and then pop closed and whatnot). Instead of replaing it, I simply cleaned it with brake parts cleaner while manually actuating the piston and once I reinstalled it, problem solved!