Hmm, well usually this would be a wheel balancing issue, but maybe you need new tyres!
Have you checked the wear lines on your tyres recently or checked for crazing and cracking?
I Have the same problem with my car at the moment as well which is a 106, as soon as you hit 70 the car starts shaking! But I'm almost certain it's the tyres.
As for your front seat, I know a lot of people who had the same problem, including myself! and I recently spent 3 hours trying to fix it myself!
turned out, maybe due to excessive shaking over the years or just a bodged repair job, 1) A Bolt was missing that bolted the sliding plate to the chair 2) another one was just loose and 3) there was a little gadget that slides along with the chair on it's runners that stops it from moving that had come loose, VERY hard to get back on! haha
The thing you're looking for is a brassish coloured piece of metal with a hookish looking lever on it and a steel spring thing that helps lock it in to place; check to see if this is actually hooked on to the chair and not just sliding itseld around willy nilly!
Hope this helps; certainly does make a difference to get these things fixed.
My passenger chair AND drivers chair were doing it, I've fixed the passenger chair but dunno if I have the strength to do the drivers chair!
Manticorp-