This should do the trick...
I had the EXACT same problem with my PowerBook G4... Everything else on the computer worked flawlessly except for streaming flash videos. I have the 12" model. 1.5 Ghz, 1.25 GB RAM.... People will tell you its the CPU and that the G4 is obsolete, it's the drivers, it's your internet connection, etc etc It's NONE of these. In my case, YES, the CPU usage goes up to almost 100%, but there is a reason for that! The wonderful folks at nVidia and their failure to properly support Flash playback on the nVidia Go 5200. The video hardware does not properly process the video so the CPU tries to compensate, resulting in slow playback and high CPU usage. If you hold down CTRL and click the video, go to "Settings", UNCHECK "enable hardware acceleration", you should notice a DRAMATIC improvement, whereas before the videos were unwatchable, and now they should play relatively smoothly, even in full screen! Hope this works for you as it did for me!