well, again that depends on what the problem is
could be a lack of ram, but could also be an insuffiient video card, insufficient hard disk speed, hard disk fragmentation, corrupted codecs, conflicting codecs, improper SETUP of the codecs, non-linear video streams, and, unless we're talking about some sort of 'legal' hd-dvd video download, there's certainly a chance that someone on the other end messed something up in the encoding process
my guess would be insufficient video card, bad encoding, just plain too slow of a computer, or a badly fragmented file if it's downloaded from bitorrent
open up the file with gspot, make sure you have all the codecs you need, and that they're all up to date, close other programs, run a 2 or 3 pass defrag overnight, and if it's still choppy, it's a hardware issue
for an easy fix, get rid of all of your codecs, and install VLC player, it's a media player with all of it's own codecs installed into itself. I hate it, honestly, but it's specifically written for hardware-accelerated HD playback, and will stream the video directly to your video card, and do an overlay by default.