weird AC problem
 

I have a 99 civic. The air conditioning usually works beautifully. I
usually drive about 11 miles to and from work, but don't use my car
much beyond that. A few weeks ago, on a hot and humid day, I was
driving on a highway for about 40 miles. The AC kept alternating
between nice cool air and hot and humid air. It wasn't very pleasant.
Since then, I've had no problems at all. What could have caused this?

Thanks!