video playback freezes constantly

I've done a lot of searches on this and found a lot of people with this issue but no real fix. I have a new Macbook pro running itunes 10 and there have never been issues before. Any video I play is choppy and freezes every few seconds. I wasted $50 in a converter that crashes and takes ages to run through so I'm back to trying to resolve it on forums. This software should be reliable. Does anyone have a fix for this? I know a lot of people have the same problem.

Thanks for any help.

=) Leah