WoW -- Black Tear and Random Black Flashing screen

I don't remember having this problem before while 2boxing and just encountered it today.

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I run two accounts, fresh install of WoW with no addons currently. I wanted to experiment with boxing software before deciding what route I wanted to take.

Currently I have video settings set to Ultra. I haven't really started to turn off any settings under video yet. First attempt at a fix will be a complete directory copy I think.

My PC is pretty beefy. Quad Core 2.8, 8 gig ram GTX260. Up to date drivers and OS. I do run two monitors at present of different size. I seem to get that black tear regardless of how I set up the two clients of WoW tho. Both on one monitor, one on each, and it usually is the first client that gets loaded that goes bonkers. For the time being, I'm content with throwing the troublemaker client on the back burner and have it autofollow while soaking up recruit a friend exp.

Any suggestions?

EDIT"

After playing with it a bit, I found that I could fix it by backing off my 2nd box vid settings in game. I did not isolate what setting it was yet, but I did correct the problem Sorry if this was a common repeat question. I feel rather silly now that I didn't realize what a simple solution it would be.

Sounds like the setting were too high even though it seems your computer should handle it. Maybe an overheating problem?

Almost guaranteed that's either a thermal issue on the graphics card or a bad bank of graphics RAM. Drivers can cause that but it's so rare with modern NVidia/ATI solutions that it's usually not worth mentioning.

I've had a similar problem - turn shadow detail/shadows off on one or both clients. May as well do both.