High RPM's while in idle

The most common cause for this type of condition are vacuum leaks. They can be open leaks like a bad rubber/plastic line or hidden ones like the intake gaskets or inside the brake booster. If outside temperature does not seem to matter in when it occurs I would go over all of the vacuum lines looking for cracks/damaged fittings or other damage. The heat and atmosphere under the hood will deteriorate the lines and elbows and they get dried and start to crumble and they will leak. Also check the air duct between the throttle body and the MAF sensor. They can crack and cause issues.

If the condition is worse on cold days and seems to go away as the engine gets up to temperature I would look at the upper intake manifold gaskets. You can test for a leak by using some starting spray when you start the engine after it has cooled off. Just start it, then quickly spray at the area between the upper intake and the lower. If it suddenly revs up you have a leak. The images show where the upper intake gaskets are. If you are going to do the upper intakes, you will be well served to do the lower intake gaskets as well as you will have everything out of the way.