Once our 84 Riviera had an intermittand window problem. After I remove the motor from the door, I carefully "uncrimped" the terminal piece at the rear of the motor (it's not very serviceable). This revealed the brushes, springs, and related connections. On one side of the brush area there was a bi-metal strip with a kink in it. Apperently it is designed to open the contact when the bi-metal heats up if someone's finger leans on the button to long. I added a slight bend to the strip to increase the contact pressure and cleaned the contact area. After testing and reassembly, the window work fine for years and was working when we go rid of the car at 186,000 miles. If you try this, be prepared to chase the brush springs.