We had the same problem with our 1986 300SL. The sun visor kept gowing down and up several inches during topless trips. The W113 SL's had sun visors that could be adjusted/tightened so that they wouldn't move by wind, the W107 misses these adjustable sun visors.
Does your sun visor suffer from the wind or does it drop down at any possible moment regardless whether the top is on or down?
with a minor 600+k