Problem with GPS/pace

Hello,

I have several problems with my Garmin Forerunner 935:

- Sometimes, the watch adjust automaticcly to a wrong hour (for now for example it says that is 22:01 as it is 10:47...)

- My GPS is false: When i run with friends who also have Garmin watches, my laps (km) are delayed more and more throughout the run, and the gap is increasing during all the run.

For example this morning, i run with my wife, we started the same time and ended the same time, we ran together at the same pace during all the run and i have a distance of 8,29 km with a pace of 5:06 /km and a time of 42min 20s while she has for the same run 8,47 km with a pace of 4:59/km and a time of 42min 18s.

Do you know what is the problem? 

Many thanks,

Greg