Ashley, The idea that a frame rate from a different monitor is used seems plausible. But why does this only happen with "Latterbox scale" mode, and doesn't happen with "Letterbox integer"?
Could it be that the resolution or aspect ratio of the wrong monitor is used in some calculations? One monitor on the video is definitely wider that the other.
fart6407 Can you try with "Letterbox scale" mode, but don't maximize the browser window? Then drag it to another monitor, and again, don't change the window size, and don't maximize it. Basically test on both monitors with the same window size.