I have a Sigma 12-24mm f/4.5-5.6 which I am very happy with.
I have a Canon 5D MkIII body.
The lens will focus normally, then after a certain amount of time will stop focusing. To force it to start focusing again, one must remove the lens and attach it again - then will work normally until it stops autofocusing again (maybe 30secs - 1 minute.
On other Canon bodies (not 5D MkIII) the lens works normally and I have no problem with other lenses on the body.
Any advice would be appreciated.