59ci merc 1.64/1 box issues

i've been having a hell of a lot of dramas with my 1.64/ 1 merc box .... its only when i run my 12 x 20p merc cleaver .
the prop has been balanced and only seems to give me grief when it its catches some air and spins freely ...
on the flat water runs fine ..
when it dose get some air it rattles the carrier bolts loose and eventually one snags the prop blade smashing the carrier mount..
my question is , there seems to be a grove in the rear of the carrier for another o ring ? this is a new box and dosen't have it neither dose my second handy?
would a rear o ring help the vibration going through the box ?
anyone had similar dramas with there 40/60 mercs?