Got it. I haven't played with PPC OSX in a very long while, and most of the videos I was playing back then were of the DivX vein.
I know PowerPC systems were optimized for codecs of their time. The very first Sorenson video codec was optimized for QuickTime on PPC Macs, whereas CinePak was the standard for pre-PPC or 68k Macs. The VCD format, using the MPEG-1 codec, played pretty well on pre-G3 and of course all G3 Macs on up. I believe MPEG-2 video, the standard on DVDs, should be pretty easy for the G3 processor to play back as well. This is where the venerable Perian and it's cool sidekick NicePlayer shine. And as I mentioned, 3ivx helped simplify DivX SD playback and eliminate that codec's issues on PPC Macs, but mostly on G3 systems. Modern MP4 is still from that era, but it taxes G3 systems, especially at higher resolutions. G4, G5 and Intel Macs worked better with MP4 h.264, and now with h.265, only modern Macs can handle that format, up to 4k. It's been quite a road for Macs and video formats!
Keep in mind my comments and suggestions are from another era, as most of our software here at the Garden is, but they may be useful in troubleshooting your video playback issues