Have not been running with 'Power management mode = Prefer maximum performance', but 'Adaptive' (by mistake), and have been experiencing a lot of problems with 'GPU Usage' suddenly spiking at what seems to be random points, along with more than halfing my FPS and making the game more or less unplayable.
Only fix has so far been complete restarts. I'm hoping changing the power management mode from adaptive to prefer max. performance will solve the problem completely, as nothing else has so far.
What's been weirdest is that I've been able to run complete Conquest runs, 20-man raids etc. without a problem, and then it would suddenly spike afterwards standing in Sanctum/Meridian, or while running a 10-man etc., so there hasn't been any red thread of any sort. At one point it spiked in Shimmersand (moving towards large areas of water does seem to spike the problem, but it's just a hunch) while the zone was more or less empty, immediately increasing GPU Usage towards max. (from a 30-50% point) and lowering the FPS to about 20-40% compared to the second before. With no fix other than restarting the computer.
This never happened with my old GFX card (GF GTX 275), but appeared since I had to change it to a GF GTX 550 Ti, so it's probably very related to specific GFX cards.