This problem has actually been happening since at least V3, which was the first version I owned. However, it does seem to have gotten worse or more frequent with V5. Moving the export window around does generally work around the problem. What I've found is that you don't need to move it around the whole time. When the export freezes it seems to always happen at around the same time on a given track. So if it freezes at 1:30 through a 4:00 minute song, you just have to move the box once every 1:20 or so to keep it from freezing. It's as if you are resetting the 1:30 countdown timer each time you move it. But, again, the specific time will vary depending on the song, so you need to look at where it freezes to know what your time limit is.
It's totally bizarre, and even more bizarre that they haven't fixed it through at least 2 major version upgrades, and numerous minor updates. Since we know that moving the export window around bypasses the problem, I would think they could just fix the problem by having the export window move around by itself. That would be a little ridiculous, but no more ridiculous than the problem itself or how long it has persisted.
