I am sorry to say the codec pack is probably the cause of this issue. Windows Vista Home Premium already has suitable MPEG2 and WMV decoders and I suspect all the dreams you are having problems with are probably MPEG2 dreams and the ones which work are either dynamic (the two I suggested), or WMV based.
From experience we have found the Microsoft MPEG2 decoder to be reliable and in an ideal world you should not replace it with a less compatible one. This is especially a problem when it comes to looping videos smoothly as some codecs simply do not handle this well and can crash, leak, or stutter.
Unlike Deskscapes 2, Deskscapes 3 previews the video in a 32 bit process (vs not showing a preview at all in v2). This uses the 32 bit video decoders. When the dream is played on a 64 bit system the 64 bit video decoder is used and I wouldn't be surprised if your codec pack did not install replacement 64 bit codecs and so it uses the Microsoft one. This would explain why v2 worked ok for you as the 32 bit replacement codecs were not used.
Does the codec pack allow you to disable the installation of a new MPEG2 decoder and just install the other codecs?
The only other thing I can suggest is using a tool to alter codec priorities and putting the Microsoft one back to top of the list for MPEG2 files.