My '93 S500 shakes when driving between 50mph and 80mph. It feels like out of balance wheels but I've had them balanced a couple of times. The whole car seems to shake - could it be the prop shaft {drive shaft in US}? out of balance. I haven't hit anything and by 100mph it seems ok again and it stays smooth and shake free at 140mph. Any ideas as its driving me crazy!

TIA Andy