The c-class just tends to be affected by crosswinds at high-speeds. But my friend's 323is was no different.
I never notice it unless going above 90, and then sometimes at 100, and at 110 I notice it a quite a bit, and at 120, i can get moved over into the next lane in a millisecond (yes, I know I am not supposed to go 120!! I only have twice in my C-class).
Are you sure it gets pushed when traveling at 75 mph? Seems kind of a low speed to get pushed by the wind, like I said, the slowest I ever notice it is about 90, unless it's unusually windy. But if it's only a few inches, then I suppose that's normal, maybe I don't even notice it? But I am always able to correct the adverse affects of the winds without any problems.
I am assuming your shocks and springs are still good?
I am sure your car is fine, that's just the way it is. How windy is it where you live on avg.?
2006 E350 w/ 155k miles (Daily Driver)
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)