wipers stop working intermittently

The wipers on my 03 Z4 stop while operating with the wiper switch on -will stop at various positions on the w/shield . They will start working again but no pattern. When they stop , turning the wiper switch to the different positions does not help and the windshield washers will not work. The local BMW service manager thinks it is the wiper motor/transmission assembly that is going out "of timing" and requires replacement. I think it is the wiper switch. I haven't been able to have the wipers not work at the garage to be able to check while not working .
Any ideas will be greatly appreciated
Thanks
WAyne

Errant Grease in Motor

Apparently there's some grease in the wiper motor which is not supposed to be there and causes the weird behavior. Check out the fix/walk thru with pictures on bimmerforums.com under Z4 (E85,86), DIY.