The solution in my car was to replace the door lock. I tried to lube the door lock before changing it. However, it helped for a little but, after a while, it did not work long term. Too bad the keyless entry was not an option back then.
You have three choices:
1. Take the lock apart, check for broken parts and/or lube it. (cheapest)
2. Change the lock ($$$ from MB) or,
3. Install a keyless entry/alarm like it was discussed in this forum before, so you will not have to use the key to get into the car. (update the vehicle)
The metal used in the lock is poor quality therefore, the problem.