Freezing and random rebooting - not during activity

My Instinct Solar suddenly started freezing.  I then reboot it (hold light button for 15secs to switch off and then turn on again).

It starts up fine but after about 10secs it reboots.  turns on again and reboots again after 10 secs.....constantly.

If i go into my activity tracking while its on, it works perfectly, tracks my activity fine, doesn't reboot during tracking.

The moment i finish my activity and go back to the normal watch face (home screen), its starts rebooting again at regular intervals.

I've tried resetting to default settings and that doesn't help.

I'm running 14.10 software on my instinct.

Thanks for any help, this is frustrating!