Vibrations on a 99 A6 Avant

I have a 99 A6 Avant that has developed a shaking in the steering wheel and the car at high speeds. It starts around 50-60 and reaches its peak around 70-75 (I can go faster than that and it just pretty much stays
at the same intensity level). I have taken it to my mechanic (who frequently works on Audis and is also a family member so is trustworthy) repeatedly and have had a regular balancing, high speed balancing, and a four wheel alignment performed. The problem has become slightly less intense, but not significantly. There are 4 new tires on the car with less than 10000 miles on them.
I don't know if it's related, but the car also has a mysterious pulsating when the wheel is turned sharply.
Any suggestions for either problem (the high speed vibrating or the pulsating while turning) would be greatly appreciated. I read something about a Hunter road force test in another post, but am not sure if it applies to this situation or not. Thanks!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Check your tires carefully for a broken belt or the beginnings of some delamination -- you may have a bad tire. This can cause a shaking of the steering wheel much like an unbalanced tire, but you should also notice a slight tendency for the steering wheel to oscillate slightly at low speeds (with your hands off the wheel). A tire with excessive radial runout will also cause your symptoms, but they should have caught that when you had the tires rebalanced.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.