Why does my shutter speed dial stop working periodically?

I have a Canon Rebel Xti that has never given me any issues except for one: Every so often (sometimes with months of perfect operation in between), I'll be in the middle of a photo shoot in TV or M mode when suddenly the shutter speed dial stops changing the shutter speed (or aperture). My camera continues to work fine in all modes as long as I don't have to use the dial. If I put my camera away for the day, it will work fine the next morning. It always happens after shooting a few or many pictures, never when I first turn the camera on. Does anyone have any idea why it does this, or if it's repairable?