choppy playback with Quicktime

I've had my iMac G5 for about a year and a half but for the 5-6 months i've noticed that Quicktime movies (My quicktime movies that i've made myself and are stored on my hd) can't be played back without them loosing frames or being "choppy" - it eventually catches up - but I may have lost an entire sentence from the person in the video - this happens with any videos I watch - not just quicktime - stuff on iTunes, recordings from EyeTV do this also - not as much - but these are usually Mpeg-2 and are recording in the best quality option possible.

any suggestions? do you need certain information about my computer specs to help?

thanks.