Choppy & pixellated playback with Live TV

I have a Revo R3700 (NVidia ION2 GPU) with OpenELEC 2.99.3 64-bit on the HDD and a KWorld USB UB499-2T twin tuner and am using tvheadend, Tvheadend HTSP Client and hdhomerun. Standard definition live TV playback is often very choppy & pixellated and occasionally causes XBMC to crash & restart. MP4 files from my NAS generally play OK but occasionally freeze. The same tuner works fine on the same computer when using Windows 7 Media Center 64-bit.

I also came across a few posts that indicated that there was a problem with OpenELEC and ION2 GPUs - does anyone know anything about this?

Log file here

Thanks