I have a 98 Honda civic LX. Having a problem with it Idling low on start up and then when gets to regular operating temp will idle high. Low idle on start up around 600 rpm and when warm around 1500 rpm. What could this be? when i let off the gas when warm it will drop down to about 1000 rpm or so but then shoot back up to 1500 and stay there. Check engine light is not on no other problems other than that. Can anyone help me?

Thanks
Matt