DoriEdd
I don't know anything about the .mpx file format sorry. Mainly it's .AMV or .MTV formats here.
When converting to .AMV files, make sure you set all the quality settings to "HIGH" instead.
Have a look at this Knowledge Base:
How to: Convert to .AMV using the MP3 Player Utilities 3.68 here. If you don't have the settings on "HIGH", you will notice the "jumps" and skips.
Also, check the FRAME RATE of your video files. If it's 30fps or higher, you might experience problems. Try converting the frames per second to 25 fps or 29.97 fps instead....