Surging idle

Is the check engine light on? If it is, then you could scan the computer for fault codes which would give you a hint as to what the problem might be.

The problem you describe has been described to me as a "rolling idle". Throughout the engine compartment there are several electrical connectors that have three to ten pins in them. Sometimes those pins become contaminated with moisture or even lose contact with the female slots they plug into. You need to unplug them, clean them out and/or bend each pin slightly (very slightly) to the side so that when you plug it back together each pin will make firm contact.

That's my shot in the dark. I hope it helps.