my video lags...

[Blizzak]

I have a problem when i play my videos on real player and windows media.. they lag every 2 secs, then catches up to the real time. for example it will play 2 seconds of a movie then pause and fastfoward to the time it is at. I did all the upgrades to windows media and i can run my pc games prefectly so i dont know if it's my video card(radeon ddr). please help with my problem