Apologies if this has been mentioned before but a quick search just brought up the problems mgf owners have with the mirror falling off the windscreen.
On my TF the rear view mirror is on a metal channel and it would vibrate quite badly, to the point where there wasn't much point in looking into it! I slid it off it's mount, applied some silicone and slid it back on again. Problem solved without taking any more drastic measures.
