Sigma 120-300mm f2.8 DG OS Focus Issue

Hi, I've been a keen wildlife photographer for many years. I own a Canon 7D with various lenses including the 100-400 f4.5 5.6. I purchased the Sigma 120-300 f2.8 DG OS lens because I really wanted a faster apperture lens for early morning late evening or overcast shots. The Sigma Lens seemed the perfect choice and with it being a zoom its very versatile too.

The problem I have is as follows. When I angle the lens more than 45 deg upwards the auto focus stops working, its as if the weight of the internal element is slipping somehow. Then I tried the manual focus ring and the same thing happens, when I try to focus from min to infinity the internal focus element just slips without any change, when I look at the distance indicator through the window it stays in the same place. Has anyone had a similar experience because when I phoned Sigma they said this is the first time they've heard of it.
Regards,
Martin,