Strangest thing -- pedaling along, switched to the middle chain ring in order to begin an ascent after stopping for traffic to clear. But when I tried to start up again, the sprocket just kept turning, with no resistance and no power to propel me forward. After a few minutes of playing around with it, I shifted back onto the big chain ring, and power was restored. I returned home without incident. This morning, I thought I felt a similar loss of power for just an instant, which then disappeared and I had no further problems. This has never happened to me in 30 years of biking, and this is a good Trek racing bike (2100) with about 3000 miles on it. Any advice or ideas much appreciated.