This started happening after I upgraded from 11.10. I'm running on a Latitude 13 laptop with an Intel Mobile 4 Series Chipset Integrated Graphics Controller (not Nvidia) – no proprietary drivers. It seems to happen under load, though I can't tell if the load is caused by something with this problem or by a program I'm running.
I'd like to try nailing this problem. Ideally I don't want to switch to something else (at least not before knowing that that's the only way out and that it's specific to Unity and not, say, X).