I am so glad to have found this site. I have a 2003 Z4 and my steering has the exact same problems in hot weather. I took it to the dealer in 2006 and they replaced the steering rack and things seemed better (but it wasn't hot.)
Yesterday it was over 80 degrees and I drove my car about 150 miles and was somewhat on edge about wether the steering would lock up as it had no correction to center, was heavy and somewhat notchy. ALos the car could barely hold a straight line wihtout constant correction. I really feel this is a serious safety issue. I'm copying a lot of these posts and will call my dealer on Monday, expecting resolution.