Usually, it's when I play for around 10 minutes. And that's kind of a pain since level 20 is long enough that the game freezes partway through.
What's odd is that the version I played on Newgrounds did not freeze.
Yeah, I really do want level 20 to be a continuous long level, since it gives you a bit of variety in the pacing. That's very strange though, that the Newgrounds version didn't freeze. Especially if it's the same build version.
FYI, I've been able to reproduce the game crashing/freezing. Seems to happen when you let the game run for a while. Will post an update when I find a fix. Keep you posted.