I am fairly to the forum, as I am reading this quote, I realize that this is happening to my IS also. I recently bought my IS 250 RWD few weeks ago.
When driving on the freeway, I sways alot like kardanial described. Driving on surface street is fine, it handles great on turns and all. I guess when it starts hitting 45 mph and up, it'll start swaying really bad. It seems like the alignment is off but the car is fairly new. I think it is dangerous at times when swaying back and forth at 70 mph on the freeway.
It can also go straight if I make it go straight but after a short distance it will start swaying.
Let me know if you can fix this ordeal.
junkjar17