Tony:

From what I have heard, the earlier 850s, including my 95 850, had lock motors that could fail due to overloading. They didn't have a way to detect resistance, as when the lock is actuated, and shut off, so they would keep running for a period, stressing the motor. Later on, at some point, they switched to improved lock motors.

I had a quote on my 850 for the front doors of about 500. It's a pain since the interior door sheet metal is in the way, as well as the glass and window mechanism. If you can live with it, then I would leave it alone. I have been living with a drivers lock that opens with the remote, but not with the key, and a passenger front door that opens with the key but not the remote. Since I know the cost and cause, I am not in a hurry to fix it.