Comment 1 for bug 572778

Given the difference seen between HD (720p) and SD video, I suspect your CPUs are not powerful enough to show the HD videos smoothly.

Can you verify that hardware-accelerated direct rendering is enabled? - please provide the output of glxinfo |grep render on each system.

Also, on the NVIDIA 8400 GST if may be possible to enable VDPAU if it is not already enabled.