the jerkiness during the playback may not be an indication of something actually in the movie itself but rather in the player's inability to decompress and feed the frames to your frame buffer in a timely manner. Some movie players have the option to 'stay on track'; with timing and skip frames as needed. You may be witnessing its ugly side effect.

Some movie players are fast at decompressing a compressed movie (i.e. applying the codec's algorithm and generating trhe pixels). Others may be slower but you may have options to use acceleration. Perhaps your viewer doesn't use them?

Then there's the choice of compressors (or codecs). The benefit of a codec is that it can significantly reduce filesize, and since disk i/o bandwidth is often the limiting bottleneck especially with large image sizes (widthxheight) of the movie clips. Often times nowadays it takes less time to decompress the compressed stream than to read it uncompressed from disk.

If you want to compare apples with apples (or with windows for that matter), you'll want to make sure they use the same codec (compressor) and have similar options enabled/disabled such as for hardware acceleration: Quicktime palyer, WIndows Media player, WIndows Media classic, Irfanview... whatever you use, don't assume they all mimic eachother.

As for the codec or compressor, it can have a big impact on the resulting filesize or video quality. If you can  find a common denominator, such as the same compressor in Quicktime as codec in avi file, perhaps something like using one of the copmmon Indeo or mpeg4 or h.264 codecs, then be sure also to give them all the same parameters (framerate & bitrate especially!)

-Philip