I have done roughly 220 hours without any issue with my 810 I was forgetful and charged it at work and it updated to 5.1 on Garmin Express and it then started freezing and turning off. Now the strangest thing was it was mostly in the morning at the coldest temp and over bumps.
I rolled it back to version 5 and that seemed to make them less frequent and now it was remembering the current ride so could restart it (but still bloody annoying). I had resolved to start the bang against a brick wall Garmin Support route (still in warranty) decided to pull out the SD card and it has not done it for 4 hours, so far.
BTW the £82 is not a repair, they will send out a refurbished one for that with a 3 month warranty.