Chris Marshall: If your saw has conventional pulleys, buy a link belt to replace the standard drive belt. It works wonders to remedy the exact problem you're having! Ironic as this may seem, I put one on a Powermatic contractor's saw years ago, and it took care of the "galloping" problem immediately. It was the best investment I ever made for that saw.

Tim Inman: I have two thoughts to share: I'd explore the effects of setting a spring tensioner on the motor. Keeping a constant pull on the belt tension will help prevent the harmonic vibrations you're experiencing. Something as simple as a screen door spring hooked up to pull against the motor would work for a test. It might even be the final solution! The best suggestion, though, would be to replace that old V-belt with a new "link" type. These belts are a little more expensive, but they eliminate the "wobbles" caused by old hard V-belts that are "out of round."