High CPU/stuttering on high bitrate X.264 MKV

[freem@n]

(12th Dec, 2013 08:11 AM)mikeemouse Wrote:  Network (wireless or LAN): 500mbit

How did you get 500MBit LAN connection? the Pi has only 100MBit. Furthermore Network and USB controller have to share bandwidth.
but back to your problem.
I guess DTS Passthrough to your AVR is enabled. otherwise it would explain the high CPU load because of decoding which can be pretty slow.
do you have any other services running in the background? (downloading/extracting/update/etc)