Freezing games seam to be one of those "grey area" problems.. It's going to be caused by different reasons for everyone, and we'll probably never figure it out 100%
It's plauged consoles since the use of CD type media..
Do a search for "freezing" on the boards and look at the efforts of others. Try all the different suggestions, and if they don't work, just investigate it yourself and see if you can come up with new reasons. If so, post it!
I'm not trying to "shrug off" your question ... it's just that I doubt anyone here will be able to give you a definitive solution. It's trial and error
P.S. If the games freezing are on DVD, then try a new disc/new burning method. Also, try the origional DVD, and see if it exhibits the same issues.