It would help to know what you are playing the videos through.
If you are running videos through iTunes read the recommended hardware. Last I checked it said something on the order of 2.0+ GHz and 2 GB of RAM, which you most likely don't have. I have two G4s, one with a 1.5 GHz processor and one with a 1.67, and neither played videos well on iTunes, but worked flawlessly with Quicktime.
I don't use quicktime, though, I use VLC
http://www.videolan.org/vlc/
VLC is the best thing ever for video playback. If you search hard enough you'll find out how to add your iTunes registry to VLC so you can play videos you downloaded through iTunes.
