If it has not been replaced ever, try checking the steering stabilizer. It's a shock absorber bolted to the center link and idler arm underneath the car. It absorbs unwanted movement in the steering linkages (esp. going over bumps, but may be helpful in crosswinds as well). If there is no resistance (should be pretty firm), then it is bad.
My 300E used to be very unstable in crosswinds, but then I had the rear suspension links and the steering stabilizer replaced. Which of the two had the most effect, I'm not sure (was not able to find the opportunity to compare before and after between the two jobs), but the car is pretty resilient to wind now.