Nivida Geforce GTX 550 ti

Hey i just bought a brand new psu and ang a brand new gpu from a local store and it appears that i have a problem, I've bought thses a 500W Thermaltake power supply and a Nivida Geforce GTX 550 ti. The exact reason why i bought theses 2 is to play Guild Wars 2. I've installed both by my self and it works very well, although i get a problem while playing the game i bought the graphic card for.

When playing guild wars 2, my game keep lowering and going up again, it goes black screen for 3 secs or freezes for 3 secs and then back to the game, i just downloaded the lastest online drivers, i think i am running direct X 10 right now and i am just not able to fix this problem, games freezes then goes back on, it happens after 5 mins of playing and then happens 2x per 5 mins...

What should i do to fix this problem?