Great software by the way many thanks

I wondered if anyone can help me with a general performance issue. I'm getting some stuttering during playback of HD movies especially during highly detailed scenes. This suggests that I'm at the limits of my machines abilities but when I check the system monitor the cpu is at 60-65% with available ram and virtually no pageing. Am I missing something?
The machine is Windows 7 64bit and I'm running over 100mps ethernet. The test transcode for a 1080p movie is holding 23fps (23.9 required) but stutters. Yet when I RDP onto the server it's quite responsive.
Thanks for any tips.
Bob