There is a circuit board that controls the function of the wiper
motor.  This board has been know to crack.  Check with your local
dealer to see if there is a recall out there.  I had the same problem
with my former 1992 Trans Sport (own since new).  The fix was a new
motor for  $ 500 installed.  However, after 11 years and 130K miles,
it was the only part that ever had to be replaced beside alternator
and water pump (that was a planned replacement.)

You can check to see if the ground to the motor is clean, the
connector plug is clean.  If the connections are no good, that can
cause it to act up.  If this checks out, then look to one of the big
box auto parts place, or the mom & pop one down the street for a new
motor.

Check the local bone-yard to see if they can get you one from their
network.  Its a simple motor with a cam.  Its the electronics add to
it that fail.

Post back with the outcome.  Good luck.