Typically when you get random jumps it is usually the IS in the lens that is going bad. the same thing happened to my old 17-55 IS. The IS in the 70-200 II is known for making awful grinding noises sometimes when it starts and stopps, maybe it is more prone to failure than in other lenses?
Sorry, been away from the computer.
The lens works perfectly fine with my 5D and my previous 5D MK II. No grinding noise and perfect IS function.
I got the lens last year (2011) in August. Submitted a report to Canon and they didn't really give me any information that I didn't already try to guess.
For the most part is works fine and stabilizes normally but once in a while it will jump, very very weird. Hate doubting my equipment especially when I'm trying to shoot a dark reception at a wedding.
Thanks for the responses, keep them coming.