I've watched the GPU Temperature while playing WoW. It seems that whenever the temperature goes over 85° the screen turns white. Then, when the temperature is under 80°, the screen turns back to normal again. I found out, that when I make a screenshot while the screen is white, I get a normal result. I don't think that this problem is caused by Wine but rather by my 8600M GT. Thanks to Cloudef and tparker anyway for trying to help.
