Problem fixed. It turned out that there is a rubber boot on the end of the throttle grip which had slid over and bunched against the bar end cap. The 100 degree heat made the rubber boot soft, and more susceptible to be worked off the end of the grip. I slid the boot back into place, and re-adjusted the clamp that holds the throttle/brake reservoir assembly. Problem solved. The throttle doesn't snap back like when it was new, but it doesn't stick anymore.
