QuickTime refuses to play certain files properly

I have recently tried to view files that were MPEG-2 files. I got a message saying that it can't play the files, and that I need additional components. I saw that the required update was $19.99. I debated whether or not to get it, but then I saw that Lion was $29.99, and it came with the MPEG-2 component for QuicTime. Rather than waste more money by buying the component and later on the system update, I decided to just update the system. But then I started having a whole new set of problems. QuickTime opens the files, but it also has this very terrible sound that's about as bad as a knife scrapping across a plate. Not only that, but if I try to skip to the part of the video I want to see, I get that infernal pinwheel you get when you have to wait. And to top it off, sometimes QuickTime just quits entirely. This happened at least 4 times in a row, with different files. It plays other fies just fine, but these ones seem to give it fits, despite my purchasing the system with the component that should have fixed the problem. Instead I get even more problems. Does anyone have any idea as to what is wrong, or have an inkling as to how to fix it?