I don't work on very many MB's, but I do work on a lot of Domestic and Asian power windows. The diagnostic procedure is to lub the tracts with silicon spray and the regulator slids and channels with something like white grease. If the problem is fixed, all is OK. If not the next thing is check the voltage going to the motor. Hook one meter lead to one of the wires at the motor plug in the door and the other lead to the other wire. Check the voltage going to the motor while raising or lowering. If you get a negitive voltage swap the leads. If you see at least 10V and it still runs slow then replace the motor. If voltage is low, check switches,etc.
I see a lot of motors that just get weak but switches either work or they don't.

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84 500 SEL (307,xxx miles)