328 Idling Issue

When I start my 328 the cold start valve opens up and the car idles at a higher rpm, as it should, until the engine is warm. Then the idle drops back down to around 1100 rpms, but only stays there for a couple of minutes. The engine then rev's up to around 2000 rpms and stays there whenever I'm at idle. Sometimes punching the gas pedal seems to alleviate the problem, but only temporarily. It will rev back up suddenly without any input from myself. I did replace the O2 sensor recently with a Bosch replacement that has seemed to work well. Maybe I need a new throttle switch, cold start valve? How about the ECU? Any input would be well appreciated. Thx!

 

Thanks.