I've found this to be true for my system as well (P4 1.6Ghz, only 16MB video card though). My Divx were all made with Codec 5.0.5 and one day I accidently clicked "Okay" to download the newer version of the Divx player and codec (5.1.1) and all my Divx movies started playing choppy. I uninstalled it and reinstalled the old 5.0.5 player and codec and all is well. Thank god for oldversion.com because the divx site doesn't provide and older versions of their software. Also, I tried the newest version of Dr. Divx with the 5.1.1 codec to make new Divx movies and they play choppy too. Right now seems 5.0.5 is the way to go (for me anyways).