From the symptom that you describe where it settles down after it has warmed up, It sounds like the IAC (Idle Air Control) which is used to adjust the rpm's while the motor is cold and until it warms up... generally the IAC will make your motor rev slightly higher than usual to help warm up the motor. Once the motor is warm, the rpm's return to their normal state. A typical indicator of a bad IAC is the rpms jupping around.... but as the previous post mentions, a vacuum leak could also cause this but my money is on the IAC