BMW Repair: 1994 BMW 325iC, germans in the world, wheel alignment

Question
I recently bought a '94 BMW 325i convertible.  I had a full inspection done (everything was ok) and the Carfax history report lists no major accidents to this vehicle.  After driving it on the highway, I became concerned and since this is my first convertible, I am hoping you can let me know your thoughts before I bring it to a mechanic.  My car (entire car, not steering wheel) starts to shake/tremble (on a scale of 1-5, I would say 3 for severity) once I exceed 60 mph.  The tires are only 6 months old and the steering is tight (does not veer to right/left) so I don't think it's a wheel alignment or balance issue.  Could I just assume that it is due to frame fatigue as the car is 10 yrs old w/170k miles?

Answer
You are most obvously used to disposeable american cars, this car should NEVER shake; not after 1,000,000 miles. It's made for autobaun speeds of well over 120mph if it were to shake at those speeds, we would have a lot less germans in the world.
Anyways, Most likely the problem is that the tires have not been properly balanced or they have split tread.  the possibility of a component on the car causeing that is unlikely. have the controll arm bushings, balljoints and trailing arm bushings checked, other than that i'd haveto look at it myself to figure it out.
-cheers