Somebody had posted a message that they had wharped rotors after only a few miles. I thought I had the same and have booked my car in for next Thursday. The dealership told me that the service manager wanted to go with me for a test drive and then I started thinking ... I cannot always reproduce the feeling through the steering wheel and it tends to only manifest itself with high speed braking, more so when I brake lightly ... this weekend I found the answer. One of my front brake calipers is missing the brake pad retaining clip. The pads must be moving around. Now these things don't usually fall off, so it could be that it has always been missing. Since I have done 4,000 miles like this I guess it's safe enough to drive to the dealer.