I have not rebuilt a Bosch starter, but from what I hear, this is a not uncommon issue.
You might get away with simply taking it apart and given things a good cleaning, and
some light lubrication - sometimes these just get enough buildup of dust and gunk
that when things expand ever so slightly from heating up, it binds up pretty quick.
The Bosch starters are known to turn more slowly and draw more current than the
Valeo starters anyways, though the Valeo's are known to lose their magnets (glued on
rather than mechanically retained like the bosch units). The Bosch starters
are definitely rebuildable/repairable.
The Nippondesno starters (can be had from Motorrad Elektrik) are the best starters, I think,
but they are very expensive.