Mine cost me $530 CDN with a similar issue. They replaced the entire IS assembly. I had a loose element and it would jump around while the aperture value would jump between 00 and whatever I had it set at. Once it stopped the images were still good but it would sometimes take 10seconds to settle down. I could also tilt the lens left and right and see an element moving while looking down into the lens barrel.
Yes, mine had the same problem, it would jump pretty violently from side to side and wouldn't focus. It even jumped with the IS being turned off. CPS eventually replaced the IS and the IS switch. It's been perfect since then. It was within warranty, so I can't say how much this would cost.
