My guess would be that you've got a problem with the box_MC getting in the way of the Mouse Rollever event, and is causing actionscript to jump back and forth between the mouse and box_MC. I had a similar problem once.
Try disabling the mouse event property of box_MC.
box_MC.mouseEnabled = false;
Excellent thinking, but sadly, the problem persists. Grr.
What do your buttons contain? You will be better off using currentTarget instead of target.. Is the box_mc in the line of fire of the mouse or does it appear elsewhere?
Hmm... well, with everything else being equal, something - a parent MC, something else on the stage, a background element, something is causing the button to lose focus and then gain focus again. I would sort through all the other elements that are nearby and check for errant MovieClips. It could be something as simple as a errant pixel that was never erased on another movieClip.
Also, run a trace on the target.name of the button that is firing off the Mouse Out and Mouse Over events, see if Action Script is jumping back and forth between two that might be stacked too close together.
Ding ding ding! Thanks so much for helping me think this through. I was testing importing the photos, so hadn't resized them. The child was added to the stage, blocking contact between mouse and button, therefore triggering roll off, and loop loop loop.
Thank you. I can eat lunch now.