The wiper motor is located under the plastic cowl panel under the wipers, and the bolts for it are inside the engine compartment near the brake cylinder booster unit.
Did some googling and apparently it can be one of the linkages seizing half way with through a rotation cycle or the motor going bad. Might take some thorough trouble shooting just to give you a heads up.
Make sure all nuts and bolts for the wiper system are actually there and make sure the linkages have grease.
With the plastic cover off, put you wipers on and observe their behaviour and listen for grinding or a the motor straining real hard. If you hear grinding coming from the linkages then theres probably a lack lube for good movement.
In addition to the above mentioned, the lack of may have put enough strain on the motor itself to weaken the motor but first try lubricating linkages.
Hope this helps.