Sudden black out on my screen.

i have been playing gw for sometime already and everything has been smooth but recently, it will suddenly black out without any warning. not a total black out though, i still can target mobs, i have the party list available but i cant see their health since the background is totally black. i can still see and use my skills. i still see snow but other than these, everything else is black.

may i know why is this happening and what can i do? there was once i was doing the dunesofdespair mission when i got this blackout and i was the monk but thankfully we got through the mission because i was healing randomly.
my graphics card is a nvidia 6600GT and i never had any problems with other games. the only way (i found out) to solve this is to press M, world map shows and i map to other places and then its normal again. but there're times i just cant leave my party or whatsoever.

thx.