Firstly, as your movies are worth so much money, I would purchase an external hard drive and back up your movies on that.
An external hard drive will be quicker than burning the movies to DVD (if your emac has a DVD burner), and are very reasonably priced now.
Secondly, have you run the
Apple hardware test CD because freezing is usually down to either an issue with the hard drive, an overheating issue, or a memory issue.
As your emac freezes after you have used it "for a while" it is more likely to be an overheating issue, but you really need to run the test.
Also, have you cleared out chaches, repaired permissions etc? How much hard drive space have you got left on your emac and is filevault turned on?