Usually this would point towards a graphics driver being out of date, but since you say that all drivers are OK, the cause must be elsewhere.
Do you have a seperate graphics card or are you using the CPU?
Does this problem happen when viewing videos both online and offline?
Are your audio drivers up to date?
I'm clutching at straws here I'm afraid. I'm also using an old Core 2 Duo laptop updated to Win7 and all videos play smoothly. What about BBC iPlayer, how does it play?
Sorry I can't be of more help.