First, check that the axel bolt, the bolt that holds the drum on the axel is tight!
If its loose at all it can let the drum move on the axel which in turn can damage the woodruff key which can damage the axel. If that happens you can get the delayed reaction sort of thing! ie The axel will turn then the key will grab and turn the wheel. Its delayed instead of instant!
Basically, thats where I'd be looking. I'd get it to the dealer and have them take the drum off the axel, check the key and where the key sits!
If the key's damaged the axel you might need a new wheel motor.
Just on edit, if it is key related, its not your fault. When the drum was installed the key may not of been seated properly or was dislodged when the drum was put on.
Last edited by Mickhippy; 06-26-2014 at 06:01 AM.