Buy a shop manual. Haynes sells them fairly cheaply, probably available on Amazon or from Toad Hall Books on the internet.
It sounds like you will need to either replace or lube and change the brushes on the motor. The manual will tell you how to do an r&r on this.
Semibodacious Transmogrifications a Specialty
1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)
"Time flies like and arrow, yet fruit flies like a banana"