Do you have to mouse set to automatically change focus (Snap To)?
Can you pin the movement down to anything else? Does the mouse move completely on its own if it is just sitting on the desktop? When it moves, does it hesitate first and then maybe catch up?
Are these all USB devices and have you checked for a driver for that? Tried a different port?
I cannot think of something that would make the mouse move in a strange manner unless something was requiring it to or an erronous signal was coming from another device. If the replacement mouse does the same thing, if you are not using the same USB port, then it must be something on your system.