I am experimentally running a dual core acer revo (ion graphics) as a stand alone core/hybrid. Graphics are running at 1080p.
With deinterlacing turned off the DVD graphics are smooth. If deinterlacing is turned on in web admin, even at the lowest setting, the picture becomes jerky. Because of this I am assuming some sort of cpu overhead as the hardware acceleration means the cpu shouldn't be taxed too much.
I will check the log files as you suggest.