It sounds to me like you might have this conditional formatting being done a field somewhere else in the form that is not shown when you tick the check box? InfoPath is applying the formatting and jumping to that field?
Have you also double checked the tab order of your fields?
I'm having the same problem. Mine happens in a repeating table and when the Doctors ID is entered, a rule pulls in his last and first name. it then thinks, jumps up and down.
Anyone got this figured out?
Mine doesn't jump up and down the page, it is more of a jiggle as you click. This only happened after modifying the form with InfoPath. It is when someone is trying to click a checkbox. Sometimes it doesn't take.