My speed sensor cuts out sometimes. I'll be doing 30 mph down a hill, and then suddenly I'm going 0 mph while the scenery flies by. Then I'm doing 35 mph. The cadence always works, although for some reason my max cadence is 221 rpm at the end of every ride. Garmins are kind of flaky, and not as reliable as they should be.

I use GPS for speed and distance instead of the speed sensor, because it's been more reliable for me. Which is a bummer.