I just replaced my RevA (had since release day) with a RevC due to the button issues (actually this is my 3rd, and had the 935 replaced twice all for the same issue..).
First thing the watch did was update to 3.9. Everything went smooth, except during a cycle following a course with ClimbPro enabled the watch reboots. Had it happen twice on 3.9 and once on 4.0. Upon reboot the 2 CIQ datafields I use are removed from the screens. So it could be related to the non-garmin fields (I'm still doing testing around this) but it seems to be more related to ClimbPro.
Anyone else had this issue? and yes, I have supplied some logs to Garmin support for there advice. It doesnt reboot during any other activity I've done (Run and Swim)