I've had the same problem, the frequency varies hugely. I usually find that when its bad I can get rid of it by doing the "Maintenance" cycle.
When it happens the entire keyboard freezes up, which I guess means, as someone mentioned, that the computer disables because it thinks there is a stuck key.
I don't think I tried using up/ down arrows first though.
The only way I have been able to "unfreeze" when it happens is to turn the computer off and on again.
Pressing control-alt-delete has no effect.