you may get some relief by applying grease to the runners and guides in the regulator mechanism but my experience suggests your motor is failing, particularly if you are hearing any noise during operation.

for the front windows, you can change the motor only but the on two back windows the motor and regulator are integral, which makes the mechanism a bit more expensive.

i have had to change all four motors in the last 18 months on my 87 car, which gives you some idea of the life span of these components.

they fail in a number of ways, most often totally, but the back mechanisms sometimes jam halfway down. Increased noise during operation is an indication that the motor is failing internally, at least in my case.

if you plan on doing the job yourself, you'll need to do a search on removing the door panels. i posted a sheet from the WIS on how to do this some time ago. changing the motor is a bit of a pain the first time but you'll get the hang of it. helps to have an extra pair of hands to keep the window glass up and out of the way while you're trying to remove the motor and regulator mechanism.

good luck

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1987 300e manual 250,000 km (sold)