It looks like our M3W's having never ending problems.
When I checked some older post I also see you (Planenut) used to drive with low Rpm's in higher gears. Not sure if this can be the cause of the problems but it could be.
And perhaps also important you didn't had the greasing hole in the housing what I read in your last post. Can there be a link with not having some fresh air (cooling) in the housing and the braking down of the rubber ? A bit similar like the camshaft belt. If I remember well I read this suggestion also somewhere in the forum.
I'm thinking about to make a small air scope on the greasing hole with some screen to get more fresh air in and the screen is there that in case the rubbers falling apart the pieces don't fall out during driving. I will be warned for troubles when I see rubber parts in front of the s
