check engine light

I have a 92 with 77,000 miles. I am away at college now so my car sits for weeks at a time without being driven. When I go home and drive it around the check engine light comes on. It seems to only be on when the car is idling. When I press the gas it goes off. I have not worried about this too much since I rarely drive it. Any thoughts on this? Could it be a moisture problem?