FPS problem on second video cards LCD

This is very odd. I just started getting terrible fps on my setup. I did some test and here is what I found...
I have a Core 2 duo 6600 2.4gig 4gig ram 2 hd win vista64.
I have 3 LCD and two video cards. My main card runs LCD 1 & 2.. The second card runs LCD 3.

I play with 1 guy on LCD 1 and two guys each on LCD 2 & 3.

I am lucky to get 15fps on my main screen if I have 2 wows running on LCD 3. If I move one of the Wows from LCD 3 to LCD 2 (4boxing) my FPS is normal. When I move the guy back over to LCD3 it goes to crap. Anyone have any ideas whats going on?
I have ATI 3870 512meg card main, and ATI x1950xtx 512meg card secondary.

You might have 1 pixel over laping another monitor which would cause alot of frame loss. Make sure both the windows are set it just right and not over lapping another monitor.