i have a 1992 300ce with 200,000 miles, i just replaced the head gasket, radiator, water pump and temp sensors. the car runs silky smooth but when i hit right around 80 mph the car kind of shakes, if i got a little faster the shaking goes away but only sometimes, after it reaches 90mph it becomes almost smooth again. what could it be? do you think its the suspension? struts? springs? oh and by the way the tires are brand new and balence and aligned. or is it something with the engine, please help. thanks

 

It sounds like a wheel balancing issue to me. At least, that's what I'd get checked first. Have them rebalanced. Check the air pressure also. If you have nice wheels, or ones with a thick outer lip, then tape weights were used instead of regular lead weights to balance them. Tape weights will fall off if not installed properly or sometimes in winter weather due to ice forcing them off the wheel.