Thanks guys - I'll give the contacts a good cleaning and see if that helps.

One other thing that occurred to me is this is the slowest lens I own - by a long shot. At the long end, it's a f/5.6 lens - the rest of my lenses are primes at f/1.8 or faster. Does the 5D have a tough time focusing with slow lenses? If so, this is good information - I'm in the market for a telephoto lens. I'll stay away from the consumer 70-300 zooms if f/5.6 bothers my camera!

The other thought I had was the filter on the lens interfering with the focus, but that's not it - took the filter off any same issue appears.