Hi all. I have a Sigma 70-200 f2.8 that's 2 yrs old. It is my cheap Nikon 70-200 as it is very well built and sharp to my eyes. However, there has been a recurring problem which I have had since I bought it off another fellow: I would be shooting with it and it performs as it should. Then out of the blue, the shutter freezes and the camera no long will take a picture. I can still focus but I just can't take a picture. This problem goes away if I turn the camera off and on again. But after a while, it may happen again, or it may not. this problem only happens with this particular lens. Would anyone shed some light on why this happens? P.S. this is the non DG version of the lens and I am shooting with a D50. Thanks for your help.