Just got my new baby the EOS 50D ( 1 week old). Recently i got the new Sigma 150-500mm f5-6.3 DG HSM OS lense. When i tried 150-500 on the new 50D all was ok until the Shutter speed got down to 1/40, at which point i got ERROR 99 on the screen. The lense works great on my EOS 400D.

Wondering if anyone knows if it is a software problem. And if yes where to get it fixed.
I am going to the Temora Aviation Museum in November and don't want this problem to arise in the middle of a flying display.