Replying to Problem transfering videos from camcorder to computer

So my friend and I have been filming a little bit while skiing, not sure what camera he is using but when we view the videos on the unit itself they play perfectly fine.  But when I put them on my computer they are very jittery and laggy to the point where you will see the person take off, it freezes and you see him land again.  The format the videos in are MPG4 and I have tried playing them in many different players with no luck.  I have also tried it on a seperate computer and the problem still exists.  Also, I have tried converting the videos to different formats and that has not helped either.  I have no idea what to try next! Anyone else have any suggestions?  Thanks.