I managed to resolve a recurring problem with my 310XT watch - could not DL courses, would not link to GPS easily, would not pair quickly etc
I deleted the entire 'Activities' history on the watch and then did a master reset.
When I turned it back on the unit was working perfectly and uploaded courses from Garmin Connect easily. It seems that the watch has all sorts of trouble when the memory is full.
Saved me a few dollars and plenty of hassle.
Try it?