Guild Wars 2

So when i am in game playing for like 20 minutes my screen freezes and i get this error saying :
—> Crash <—
Assertion: fabs(upMag – 1.0f) < 0.01f
File: ..\..\..\Engine\Gr\GrTrans.cpp(1431)
App: Gw2.exe
Pid: 964
Cmdline:
BaseAddr: 00400000
ProgramId: 101
Build: 16597
When: 2013-02-11T16:07:24Z 2013-02-11T17:07:24+01:00
Uptime: 0 days 0:15:30
Flags: 0

can some one tell me please how i fix this?

sinthoras did a new graphics card fix the problem u have because im having the same problem..