flashing second screen

i bought a second screen and i am experiencing some annoying flashing while multiboxing. what is strange is that only portions of the screen are flashing: i have 4 toons displayed on the second monitor, and one of the toon-screens will start flashing and i have to move it to the first monitor and back to stop the flashing. clicking on the aero task miniatures helps too. anybody had had the same experience and found a solution? i raised the voltage of the geforce 570gtx lightly, but it did not work.
edit: sometimes the flashing starts on the first, main screen.
edit: i am not sure , but i think it came because i had my main screen on hdmi, while the bios knows only dvi. windows switches screen so you only notice it at startup. i switched the cables (had to add one for the sound though), hope it helps.
last edit (hopefully): did not help. tried both screens on dvi. 5mn no problem. is that the final solution? still curious about what you guys know about this.

well, i am afraid nothing worked. and i changed the cables just to be sure. so i am wondering if it is not an isboxer/innerspace issue.