I had exactly the same problem on my aluminium bike. I had someone hold my bike and had a look fom +- 10m behind the bike. The front and rear wheels didn't allign. The vertical allignment of the wheels wasn't the same. After a bit of a struggle with the manufacturer, they sent the frame in for tests. The test results showed that the frame wasn't straight, and they are replacing it for me at no charge. Don't take "no" for an answer! Good luck.
