Has anyone else experienced this problem. I'm curious to see if its just me!

As I mention above, I have the 20d with a Sigma 24-70 F2.8 DG Macro - the lens is just 4 months old or so! Recently, on switching on, I have noticed that on occasion, I get no f stop reading on the camera LCD and auto-focus fails to work. To rectify this, I switch off, remove and re-mount the lens, and switch back on and all is fine again.

This has happened 3 out of the 4 times I used the camera last. Anyone else seen this?

Thanks,
Paul

It has happened to me before, but I dont remember which lense I was using.....I think it is just not mounting up properly or is slightly loose on the mount which causes the error.....

Nope, I just took 5000+ shots this past weekend at a cheer comp without an issue with the 24-70/2.8 macro and 20D.