Intermittent Windscreenwiper fault

[Nimrod]

Not sure whether this is mechanical or electrical, so I'm posting to both!
The wipers switch on OK but at random intervals they stop at whatever position they are in, and again at random intervals, start up.
Is there any way I can do a definitive test to find out whether this is an electrical or mechanical (ie motor) problem, without getting another motor? (Expensive!)

thanks