My rear wiper has started acting up recently, in that it will sometimes do a full wipe and return to the parked position although somewhat slowly at times, sometimes it stops at the 3/4 wipe position so I have to get out and manually move the arm to the end of the wipe arc then it will return to the parked position.

Is this a sign that the motor is on its way out or could it be the plastic gear wheel at fault. I can get a replacement wheel off ebay for £5.00 but a replacement motor (second hand) is £100+.
I'm gonna try this weekend and get the motor out and have a look, but thought I'd ask first if anyone has had this same problem.
Any thoughts as to which it could be?