There have been lots of other guesses, so here's mine:  Is there something
wrong with the front main bearing where the engine oil pressure is getting
into the prop control.  Like a spun bearing?  Then the governor would reduce
the pressure until it got lower than the engine oil pressure at which time
it can't go any lower, holding the prop at a pitch higher than the
mechanical stop allows.  I don't like my idea as it is expensive, but it
might be a logical explanation.  Could put a pressure gage on the prop
governor line if it is a rear-mounted governor.  Adjusting the oil pressure
down would then help, not hurt the problem.  Try adjusting it down to maybe
50 psi for a test?

Gary Casey