Random black screen when phased/death effect on single pc dualbox?

Hello, im using a single computer to run two WoW games at once. I noticed once in a blue moon I get it in Dala (like one toon in dala and another somewhere else, I cant have two in dala or my pc goes to .1fps forever and I have to forcequit/reboot) and when im phased for a quest like the Bonfire in Fjord or going in invisible on my mage (even the death effect when I had it turned on) that the screen will just go black and my pc goes really really slow with like less then a frame per second (I cant even alt+Tab or do anything) until the duration is gone or I remove the player.

It only happens once in a while tho, not all the time. Its completely random and I cant seem to turn off the invisible for mages or phasing for quests (the phasing type that arent cause you completed the area or havnt done something, the ones like the bonfire quests in Fjord). However it really impairs on things like if I go invisible on my mage and I cant help my priest to not die cause of this.

I have a 7900GT Graphics card, AMD X2 4400+ dualcore CPU, 2GB ram and running at 2 1680x1050 resolutions (1x multisampling and I have minimal lag when playing without those effects, I think its around 200ms). Is there a way to disable effects like Invisibility without imparing the effects like normal spells or is there a explanation of whats happening? Thanks