High CPU/stuttering on high bitrate X.264 MKV

[mikeemouse]

Software
XBian version: 1.0 Beta2
XBMC version: 12.2 Git:20130916-091cb29 (Compiled: Oct 2 2013)
Overclock settings: High (tested - stable)

Hardware
Power supply rating: Nokia AC16X 5V/1A
RPi model (model A/B 256mb/512mb): B/512
SD card size and make/type: HAMA GOLD SDHC CARD 8GB CLASS 10
Network (wireless or LAN): 500mbit powerline (actual throughput tested using dd over SMB on Pi at 90mbit). SMB and NFS both tried, both show the problem. Server is Windows Home Server, CPU load ~3% while serving files.
Connected devices (TV, USB, network storage, etc.): Pi -> Yamaha AV receiver over HDMI -> TV

Logile
Link to logfile(s): http://pastebin.com/ZAmi2bjz

Problem description:
Playback stutters during playback of high bitrate files (only MKV tested). Lower bitrate files don't cause problems.

How to reproduce:

Play high bitrate mkv. Example:

Star Wars Blu-ray remux mkv
Video: AVC High L4.1 avg bitrate 33mbit, peak 42.1mbit
Audio: DTS 5.1 1509kbit

---

config.txt

#initramfs initramfs.gz 0x00a00000
gpu_mem_512=256
gpu_mem_256=128
initial_turbo=1
decode_MPG2=<deleted>
disable_overscan=1
hdmi_group=1
hdmi_mode=16
disable_splash=1
arm_freq=950
core_freq=450
sdram_freq=500

hdmi_force_edid_audio=1
over_voltage=2
decode_WVC1=<deleted>