Monitor Flickering with 2 screens?

Can anyone help! I used to use my 20" Lcd monitor as well as my lg 50" plasma tv they worked ferfectly untill i changed my mobo and cpu and went back from Vista to XP MCE ever since i have done this im getting a flickering wave effect on both screens but when i unplug the 50" tv the monitor works fine i have tried apllying different refresh rates and resolutions some are better that others but still not gone any reason why?

My spec is
XP Media Centre Edition
q6600 2.66 Quad Core 8mb L2 Cache 1066FSB
3Gb Dual Chl DDR2 RAM
Gigabyte GA-P35-DS3L 1333FSB
250Gb WD Caviar HDD x 3
320Gb Barracuda HDD
80Gb Barracuda HDD
Geforce 7300le 512Mb Pci-e DVI-out