Tim- 
If you're borderline nuts like me, you wash your engine.  Maybe
alot. I've had problems with Bosch starters getting water in the
solenoid and not working when cold.  Either it gets a little frosty
in the plunger chamber or it washes some oil/gunk laden water into
the solenoid chamber and gums things up when cold.  Take the
solenoid off and buff up the piston and the chamber.  I like to use
'Simi-chrome' on the brass liner of the bore to make it sparkle and
leave a coat of protectant.  Don't lubricate it, leave everything
dry.

It could also be any of the things you mention.  Many of them related 
to washing the #$%@ engine.  I can't help myself, I still wash it off 
but don't use high-pressure anymore.

-dan