So we know more about it...
At the Apple Icon at top left>About this Mac, report the version of OSX from that window, then click on More Info, then click on Hardware> and report this upto but not including the Serial#...
Model Name: eMac
Model Identifier: PowerMac6,4
Processor Name: PowerPC G4 (1.2)
Processor Speed: 1.42 GHz
Number Of CPUs: 1
L2 Cache (per CPU): 512 KB
Memory: 2 GB
Bus Speed: 167 MHz
Boot ROM Version: 4.9.2f1
At the Apple Icon at top left>About this Mac.
Then click on More Info>Hardware>Graphics/Displays and report like this...
NVIDIA GeForce 7800GT:
Chipset Model: GeForce 7800GT
VRAM (Total): 256 MB
Vendor: nVIDIA (0x10de)
Device ID: 0x0092
Revision ID: 0x00a1
ROM Revision: 2152.2
Resolution: 1920 x 1080 @ 60 Hz
Depth: 32-bit Color
Core Image: Supported
Main Display: Yes
Quartz Extreme: Supported
Status: No display connected
If your Mac is a PowerMac G4, it can't run OS X 10.6.8. This version of OS X runs only on Intel Macs.
Any hoo, if this a PowerMac G4, even with a powerful video card upgrade ( 256 MBs of VRAM or greater), G series Macs are going to have trouble with online Internet video. There are more video codecs types that Internet video is compiled in that older PowerMacs can't handle well or won't at all.
This is the reality of still owning and using a 10+ year old Mac.
There is no fixes that will solve the choppy video issues.
If it's possible, instead of streaming the video, if it's a video that can be downloaded onto your Mac, it may play better that way as you are playing the video file locally off your computer instead of trying to keep up with the streaming data and/or trying to keep up playing a video codec that the G4 struggles with.
If you are trying to watch YouTube videos, there is a YouTube video downloader called MacTubes that will let you download the video locally to your Mac to play locally.
Other than this, there is not much you can do about issues with Internet video.