Guild Wars 2

I have to report frequently massive FPS drops. Normally around 70 FPS, but every few minutes they drop from one second to another down to 5-7 FPS. No matter if I run around alone doing nothing or in fights. The low FPS phase lasts some 10 seconds and then they go back to normal without any visible reason.
Strange: bigger events always produce these FPS drops, but here the same: after 10-20 seconds I have good FPS again, although there are many people and skill effects. Here the FPS drops occur more often, short time after FPS are back to normal they go down again.
I play with “automatic” ingame graphic options.

Geforce 460 GTX
AMD Phenom II X4 965 (Quadcore) 4x 3.4 GHz

I first played with a older driver, then I updated to the latest official driver, but the game said I should try a July beta driver which I did and now I got the latest beta driver from August 27, which in its description even mentions GW2 ^^
All drivers produced FPS drops.

My friend got the same GPU but has a worser CPU, and he never has any FPS problems.