What makes my X5 dance?

[Clutch]

Hi all. I finally got my first long road trip in my 2006 X5 4.8is - Washington DC to Buffalo NY and back on a variety of road surfaces, but mainly good old PA & NY interstate. During these combined 16 hours, I think I have confirmed my X5 dances across the road, and I am pretty worried about it.


The car has a lateral (side to side) sway that comes and goes. I cannot reproduce it at will. The effect can be quite dramatic at times (verging on dangerous) with the vehicle pulling quite strongly left or right. The car can be all over the lane with a mind of its own. Note that when this happens the steering wheel is not being pulled or turned – it stays straight but the whole car just veers off. This is my first car with all wheel drive, DSC, and air suspension. However, our 2005 X5 4.4 drives perfectly normal. I have wondered if the rear tire tread is acting to pull the car? It is pretty radical tread with strong diagonal grooves that I think may grip just the right way and pull the car sideways. (I have included a picture.) I bought the car with just 22,600 miles on it, and yet it had non-factory rear tires (Toyo Proxes) but factory front tires. I don’t know why they were replaced (under warranty maybe?) I now have only 26,400 miles.

I took the car to the dealer (under warranty until 10-15-10) and they said the problem was "not found". However they admitted their test drive was limited and maybe I should take the tech out for a long drive. I do not think it is due to grooves in the road - at least non ethat I can see or notice. It happens in a variety of different places and speeds.


I did a search and found a similar post from 9-23-09, but no definitive answer.


So my question to the experts is: "What can be making my X5 dance like this?" Is it the radical new tread on the rear, the air suspension, a suspension problem, or something else broken somewhere? I'm at wit's end but strongly suspect the tires. I'd like to get this figured out before the warranty expires. A tire swap test would help, but hard to arrange with someone. Any advice or comments you might have would be greatly appreciated. Many thanks in advance.


PS - BTW, I got an amazing 18.6 MPG with two adults, baggage, AC on, and running 80 MPH almost all the way. I thought I was dreaming but checked the math. I am impressed!