Very likely it's the fact you've got 2 cards running at different resolutions. This is interesting
What you ought to try now is manually forcing Vizard's resolution (and perhaps position). You can do this using the
viz.windowsize(H,V)
viz.windowpos(H,V)
commands. Hopefully these will solve the problem. I've never actually tried a 3 monitor / dual card setup but I will do so if you're still having trouble so we can solve this.