The only thing I would suggest is replacing the starter bushing.  This is the
bushing that is in the transmission bellhousing which holds the tip of the
starter centered.  If this is worn out it will cause premature starter death.
 All of the Bosch rebuilts I get (except for Vanagon) come with new bushings
in order for you to replace this every time you replace the starter.
Next time you replace your starter (not going to say if with your story)
check this bushing out with a drill bit that is the same size as the end of
the starter shaft.  If you find it has some play, drill it out and tap a new
one in.  This may be the end of your saga.
Hope this helps.

Ken Wilford
http://www.vanagain.com
John 3:16
Phone: (856)-765-1583
Shop:   (856)-327-0027
Fax:     (856)-327-2242