Got distracted for a while, but I'm back.
When I was browsing the forum before posting my original message, I saw some
of the posts about random hangs, but I didn't look into them. Since I could
reliably reproduce my problem, it didn't sound random. Oh well.
Thanks for the tip about the beta drivers. But by the time I got back to this,
there were new driver releases out, so I just went with that. Driver version 1.0-5328
seems to fix the problem.