I had something similar on my 2004
MC. I traced it to the manual locking button sticking on the door trim (that it passes through). Guess the clearance changes over time or the door trim may have been removed for another reason (squeeking window actuator!). Some oil and repeated manaul lock/unlock cured the problem. The motor does not seem to cope with much resistance. Worth a try - cheaper than replacing anything.
Herbie