I've just had to install Windows 7 on my Media Center machine because my Vista installation had a faulty drive.

I've got the latest drivers that I can find - Intel 945GM integrated Graphics, Realtek audio drivers.

Things are working OK with one exception.

Playback of old recordings, from dvr-Microsoft format files, is choppy. The picture freezes for a fraction of a second, then quickly catches up. The sound is uninterrupted and doesn't pause.

These freezes happen once every 5 seconds or so. It's very regular.

Playback of Live TV from the digital tuner is perfectly smooth. DVD playback is perfectly smooth.

As an experiment, I used the MPEG editing package VideoReDo to create a small test file in three different formats. This program takes the raw MPEG streams and repackages them into the desired container. I took the same clip and created three files in three formats: dvr-Microsoft (Microsoft's old recorded TV format); mpg (standard MPEG); and ts (raw MPEG transport stream of the kind often produced by PVRs).

When these three files are played back under Windows 7, the mpg and ts files play smoothly, but the dvr-Microsoft file stutters.

The last piece of data I have is that two other Windows 7 machines can play back dvr-Microsoft files smoothly with no stuttering. One is a netbook, with less grunt than the media centre. So there must be something specific about my Media Center machine that's causing the problem.

Does anyone have any idea where I can look now? I don't know much about AV software, codecs, filter graphs etc. but I suspect that's where the problem lies. Rendering the video isn't the problem, but extracting the streams is. How would I go about diagnosing the problem?

Edited to add: I just used the GraphStudio tool to look at the filter graph on the offending PC. The filter graph it uses by default for dvr-Microsoft looks identical to the other machines, and, interestingly, when I play the files using GraphStudio they run smoothly. Under Windows Media Player and Windows Media Center they stutter. I'd like to see the filter graph for Windows Media Player but GraphStudio won't show it. It looks like Windows Media Player and WMC are using a different decoding path to GraphStudio.

Edited again to add: Today I purchased a new HDTV. The same Media Center driving the TV at 1080p is now playing back the old Recorded TV files smoothly, without stuttering. So whatever the cause of the original problem, using a different resolution seems to have removed the problem. It might also explain why nobody else has had this problem. I doubt many people use Media Centre with a 14in portable TV.