Intermittent Vibration above 75MPH

[lietuviai]

I've been doing a lot of highway driving lately with my '85 300D and when I get up to and over 75MPH the car starts to vibrate kind of hard but not in any constant way. It comes and goes with no real predictable frequency. Below that speed its smooth as glass. I don't think that it could be the tires since I would think they would start vibrating way earlier and with a predictable frequency. I know that its not the road surfaces either. I don't think everyone pushes their W123 that fast that often but I would think these cars are capable of going a bit faster without any problems.
Any ideas?