I don't want to muddy the water here, but while the GSC-10 is the primary source for speed, it appears that the most recent firmware update may have introduced a bug that causes speed fluctuations when the GPS signal is weak...which is contrary to how it's supposed to work.

You can find out more about this at the Garmin support forums:

https://forums.garmin.com/forumdisplay.php?f=219

That said, I have the 500 and have been very happy with it. Although I'll admit to lusting after the upcoming 800...