I had a problem like that a couple of weeks ago. In my case the motor was burned so i replaced it, worked for a day and the problem came back. Took the car to my mechanic and he discovered that the wiring of the motor had been pinched on a previous air conditioner evaporator replacement. The wires loss their insullation and were making contact with the chassis. The funny thing is that they did not blow any fuses until my mechanic started to work around them (that costed me an extra motor since it burned also the first spare that I installed). Fixed the wiring and put another motor, no problems since then.
Have your wiring checked, also try replacing the wiper control module and it would not hurt taking a look inside the wiper motor. Its case has two screws that can be easily removed, look for overheating evidences.
Check this site if you want to know more about the control module:
http://www.unofficialbmw.com/e36/int...ol_module.html Good luck.
jg