if you like me have this problem, i found out (after the 3rd replacement) it is because the newly designed crap battery has got "play" and looses contact with the phone when ie is shaken sideway or from a fall (dropped times and times again before the n80 - never went off)
my solution ideally would be to return it but cannot live forever waiting for the dhl man, so i just put a bit of paper folded on the right side (if you are facing the back of the phone ) in order to avoid the play.
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