'91 420SEL trans won't shift out of third sometimes
 

Any ideas?

Kevin

Could be a valve body problem or internal, a leak in the K1 drum. Do you have 4th gear, what is the mileage on the car, condition of the transmission fluid, does it smell burnt?

What I have observed is
 

that the car has fourth gear it just won't always shift into fourth. You can manually shift it SOMETIMES. I did go from drive to neutral while on the road and revving the engine and it would shift into fourth but not every time. The fluid looks nice and red with no burnt smell. I appreciate any and all help.

Kevin

Long distant diagnostic is just an educated guess. I say you need a rebuilt valve body. Let a shop with MB TRANSMSSION experience diagnose the car for you. Not just any shop.