Had that happen to me once with my Edge 500. All of a sudden I was riding more than 100 mph. Next ride it was back to normal.

No help, I know. But just want to let you know you're not the only one.

Check out the Garmin Users Forum
https://forums.garmin.com/forumdisplay.php?219-Edge-500

I didn't find anything specifically that helped, but it's a good resource to have.

Good luck,

KB