This problem is occuring in two separate areas of my page in IE6 only.
But to keep things simple, I've just made a test page up with one of them.
When you roll over the links the whole thing shifts, and then when you roll over the button it shifts back again.
I guess this is something to do with IE not liking hovers and zoom:1, but I have put that in a separate IEFixes.css file. No luck though. Not sure if it's relevant but I also have a hover behaviour attached to the body class.