i assumed this was to do with my plan bandwidth which is limited to 256 kb/sec
Sounds right, this is very low for internet video and as bandwidth is increasing (though Australia lags here) the videos are getting bigger and heavier as well. But even if your speed is good enough sometimes the video site is slow, or there is other congestion. There are many times I just have to pause the video and let it keep loading if I want to see it uninterrupted and I have about 6x that speed (which is still agonizingly slow compared to what my American friends have, which is still agonizingly slow compared to what Japanese have....) so it might not be something you can always fix.
i recently viewed a video on this site http://www.sbs.com.au/rockwiz/which
offeres a choice of bandwidth. i selected the 256 kb option and the vid plays flawlessly. if i select "auto" it stops and starts.
The auto options often do poorly at lower speeds and many times these days sites just don't scale down low enough. As bandwidth gets cheaper and faster the smallest video size available is going higher and higher. That site still offers such options but it's going to be less common going forward (maybe Australian sites will lag due to the frustrating bandwidth issues there).
thinking back 2 or more years i did not have the stop start problem either (occasionally maybe)
I notice a lot of people feeling the same way in my online readings and my experience is similar. I think some sites may be slowing down (I see a lot of accusations that Youtube is slower) but it could also be ISPs intentionally throttling types of traffic (which is what the whole "Net Neutrality" debate in the US is about).
I used to have a gigabyte raedon video card on this machine but it seems to not be working.
Could the lack of a specilaised video card cause this problem?
It can cause choppy video but not choppy video that is choppy because of the downloading. So if it is doing this even if the video is fully downloaded then it might be the computer's local resources (I consider this unlikely) but what you describe sounds like a simple matter of a big video going through a small pipe.
Is there any adjustment i could make to assist sites to recognise my limited bandwidth to alleviate this problem.
Not really, they need to have lower quality versions of their videos available for anything to work. I suggest doing what I often do, and start the video, immediately pause it and leave it there in a background tab for a while to buffer before starting it.