I will just throw this out there as i had the same issue with my WH2O resetting.
Most people would think this is a no brainer silly mistake  but i never thought the screws that hold the board in place actually earthed the board... i thought they were just to rest the board on... it was only after multiple pages of reading on this site someone said " tightened the screws and has good earth"..... Bingo.... tightened the screws and no re sets ever since..... may be your case too... screws come loose and its not a random reset its just when the board looses the earth and turns off.
Hope this helps