Neil, I believe the problem with mine involves the frame height or aspect ratio of the mpeg. At the time those were created, The MasterBaron and I were encoding these for 16x10 aspect ratio monitors. To do that with TMPGEnc4.0 Express, we had to encode at an odd resolution, 1792x1120.
While the prior versions of Windows played these fine, Windows 8 seems to have an issue with them. For the record, these are working fine on Windows 7 with DeskScapes 8.
Since I'm going to want to run some of mine on Windows 8, it looks like a few of them will be getting an update.