Another thing you might consider is checking the housing of the wiper
motor itself, ESPECIALLY if you've recently had a paint job.  I had a
"somewhat similar" problem with my power antenna after I had my car painted.
Seems that the dealer cracked the housing when they replace the antenna
under warranty, but failed to inform me of that.  I found out a few years
later after getting the car painted, when I replaced it myself, and found
the assembly packed with sandblasting sand.  NO WONDER it wasn't working
right...

 If there were any chips or open cracks and you had your car painted, and
you know it had been sandblasted, that MAY be something to check into.  It
should be visible to the naked eye if it is a contributing factor though, so
if you can't see an OBVIOUS crack or opening in the housing, you can very
likely disregard this possibility.