It looks like a graphic card/driver issue. Maybe. Can you give some more information about your system? Mobo, processor, RAM and video.
I agree..i have the same video card as the OP and have noticed stuttering with several online video sites which never used to happen with the radeon driver that came with the older pclos2007.The way i solved it in the current pclos was to add the line (without the quotes) "option DRI False" into the /etc/X11/xorg.conf file under the device section:
VendorName "ATI Technologies Inc"
BoardName "ATI Radeon X1950 and earlier"
Option "DPMS" Option "DRI" "False"
It's worth giving it a shot and see how it goes.