Stuttering video solved (in case it helps anyone else)

After pulling my hair out for many hours I think I have solved my stuttering video problem.
I post here in case it saves anyone the hours that I put in to solving it.

Playing through an HDMI output from my PC to my 42" Plasma screen (via a Sony AV receiver)
Vista Ultimate 32 bit
PENTIUM DUAL CORE 3.4 GHZ, 2GB DDR2
The graphics card is an Nvidia 9500GT with a HDMI out.
A high bit rate 1080p ts file pushes the CPU to a max of 90% so I figured it wasn't the CPU causing the problem.

To cut a long story short, If I used a saved profile in the Nvidia control panel to set up the TV (i.e. one click and it sets up the resolution etc on the TV) it causes problems the video stutters and is choppy. If I set it up manually each time it works perfectly.

I have no idea why this should be so but having seen that the files played just fine on the PC monitor and sometimes on the TV (and then the same file would not play properly on the TV the next time I tried) I figured it had to be something to do with setting up the second screen

I am using MPC Home Cinema to play the files, which works fine with the Core AVC decoder or with its own decoder.

As a side note I found that enabling hardware acceleration to make the graphics card do some of the processing work also makes the video choppy but only on high bit rate 1080p ts files, even though it drops the CPU load dramatically. On all other files it works fine but I leave it off now anyway.

If that helps anyone else not have to go through the process of finding the cause of their problem then the job's a goodun.