This particular issue is a bit complex to articulate, so I'll begin by providing the code and then proceed to elaborate on the problem as best as I can. I'm assuming that you've already compared the results in Firefox and other browsers.
Here is the code: http://jsfiddle.net/MEckv/ (click on the big box to get started)
One strange observation is that it appears as though the left positions of the boxes are being reset before they appear, giving the impression that they are continuously cycling on the left side while only the right box changes color. Interestingly, this odd behavior does not manifest in Firefox.
Do you think there could be something unusual happening in the code?
Appreciate your assistance in advance!