1996 Chevy Cheyenne windshield wiper problem

The windshield wipers
on my '96 Chevy Cheyenne are only working occasionally. Generally, they won't work right away when I start the car after it's been sitting overnight. Sometimes they will then work after 30 minutes of driving. I've also noticed that if it's only been parked for a few hours and then started, there's no problem with the wipers either.

I'm not sure if the problem is with the wiper control module or the wiper motor. When I turn the wiper controls, even though the wipers don't turn, I can hear the wiper motor turning. In fact, as I move through the range of wiper speeds, I can hear the motor changing speeds, but no wiper movement.