It sounds as if you are having a timing problem. There are a set of switches inside the sunroof motor. They stop the sunroof at the right time. You have also a clutch in the sunroof motor. If the sunroof has too much resistance or hits the end of its travel, the clutch will slip so nothing is damaged.
1992 500E (very soon I hope
1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM