online full screen video glitch

sometimes when i watch full screen video over the internet the video gets scrambled but the sound keeps working, then my computer locks up, and i have to reset it. my computer specs are as follows.

asus p5n-d 750i
2x 1gb corsair xms2 in dual channel
evga 9800gtx
500gb hd
intel e8400
windows vista home premium

someone told me once that the 750i chipset has this problem but that doesnt make sense since its only over the internet.

does anyone know why this happens or how to fix it? i have the latest n-force and graphics drivers. the glitch only happens when i watch full screen video over the internet. it never happens when i watch dvds or video on my computer. since it doesnt happen when i play video games i dont think its a problem of overheating.

i just switched from internet explorer to firefox and i havnt had any glitches yet but ill post if i do. i dont know why a program would cause that but thats why im posting here.