My '96 W210 E320 had the same problem since new, which baffled the technicians, because the bulb and everything else seem ok, and they couldn't figure out what it was.

It turned out that the rear light module of the W210 had a common bad-contact problem. To this day (14 years later), the problem still comes back once in a while. But the fix is simple: just switching the light bulbs from one side to the other seems to help. It also helps if you clean the metallic area around the hole where the bulbs are inserted with sandpaper.