Just ran into the same problem on 2 -124's recently and a 126 about a year ago.All 3 cars had perfect flex disc and vibrations at high speed.Found loose or slightly loose bolts holding the disc on all three. So just cause they look good give them a try.They may feel good by hand but put a socket on them and give it a try.The 126 came back a month later with the bolts loose again.Guess that one needed all new lock nuts or both flex disc.

My 300CE is driving me crazy with a vibration that ,after doing every thing i can think of,vibrates at 70 and up.Not real bad but i like a Benz to drive like a Benz.I think on the coupe its the rain grooves cut into the freeways in South Cal...On smooth roads it really smooth but on grooved roads it starts to vibrate and some roads are much worse than others.I am fairly certain its the brand of tire as Michlens don't vibrate and i got cheap and bought el-chepo's
Good Luck
Rich

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88 300E White, sold 9-04
88 190E Black 2.3 sold
90 300CE White, Grey sold 12-24-04
92 400E White,Grey Sold 5-06
90 300CE 91K Signal Red,Black Sold 2007
1989 300CE Smoke silver,Tan Sold 1-08
1993 300E 3.2 M104 motor
1991 300CE White,Grey
1994 E420 beautiful car,gone to new home
1994 E320 Smoke an Parchment.Beautiful car