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).
This appears to have ceased happening at some point recently.