[bug] graphic error in some dungeons

I noticed some time ago, strange graphical issues when entering caves (there: graveyard cave) in a party... was blamed on GPU overheating then.

Situation there was, card running fine over hours even with a max. temp ~70°C due to open case from some maintenance; framelimit used and working.

Symptom 2: leaving the cave instantly by portal solves the graphic problem, but ends with a client crash on the next instance access ("programm doesnt work any more and has to be closed" error on W7/64 (translation from german error message)

Now, with closed case, I never have temp's much over 55°C, so I never expect problems from an overheating GPU, but run into a similar issue in another dungeon again - this time in the church: http://dl.dropbox.com/u/37186885/screenshot-0134.jpg

Symptom 3: it happened only in multiplayer yet, and directly on area entry, just before doing any activity that may cause troubles (no activation of auras at least on my own char, no traps, no skills, no enemies in sight...)

Even if this may be a very rare thing, would be good to have an idea for the real reason.