Try this:
http://www.broadbandreports.com/tweaks
This too:
http://www.adslguide.org.uk/tools/speedtest.asp
Do the speedtest first, then do the broadband reports tweak test and follow the recommendations, then try the speedtest again.
Also... can you ping something like say
www.nildram.net and see if the pings are stable?
I'm thinking line fault or contention but we'll see