After driving on 75, got off and was waiting at the light and CEL pops on. Car begins to idle at 1600rpms(and fluctuates every so often) and rpms drop fast between shifts. So I go to AutoZone, and they say its throwing p0505. When I went back to my car, and drove away, the light was on, but car drove normal until about 5 minutes later--then it started driving the same way.

whats wrong?