Hi, I have this problem with HD YouTube videos where it lags when it is downloading the data but stops lagging immediately after the buffer is completed entirely. I have a pretty fast laptop with an i7 processor and 8GB of RAM so I guess it is not my computer that's slow. Can this be caused by the YouTube videos buffering too quickly for my hard drive to cache since I have a 200Mbps Fibre connection and when I check the video details while HD videos are loading, the download speeds are fluctuating around 170Mbps?
If the gpu is older than the 5xx series then try forceware before the 300 ones. Try 296.10 and try using the latest flash.