I have a Thinkpad laptop W510 running Linux Ubuntu 12.04 with NVidia drivers, connected to an external 24" Eizo screen via displayport. The screen behaves ok under normal conditions, but if the CPU usage is pumped up to maximum, the external screen starts flickering every 30 seconds, pretty regularly, only once (that is, it's not a constant, repeated on/off/on/off flicker, it's a short single burst that may appear as a dark bar somewhere, lasting a fraction of a second).
If the CPU usage goes down to normal, no problem. If it climbs up again, it starts again after a couple of minutes. Cabling is perfect.
Where is the problem, and how can it be fixed?