Garmin Forerunner 920 XT Speed and distance incorrect in bike mode

During my last few ride, I'v noticed my 920xt was recording the speed and distance incorrtly. Since then I upgarded the watch firmware, but no joy. As I'm cycling the speed keeps fluctuating up and down and it gives a total distance incorrectly. It seems to only happen in BIKE mode. The route is recorded accurately though, so GPS must be working.

Any suggestions would be very helpful.