Idle issues

Ok, I just bought this 89 Accord a few months ago. The only serious problem I'm having is when the car is first started in the morning it revs up to 4000rpm until I get it on the highway for about 10mins then it seems to act just fine. I changed the plugs and air filter but that didn't fix the prob. Also if I shut off the car when its not fully warmed up it will still run for about 15sec. Can anyone help??? :banghead: