To say speed test sites can be inaccurate is true, they can be, but so can file downloads with caching and other issues.
Our tester at http://www.thinkbroadband.com/speedtest.html
now gives you a graph of the throughput during the test, and uses a single thread (like a download) and multiple threads to flood a connection. The graphs if nice flat lines show a good connection, a wobbly set of lines suggests issues.
Registered speed test uers can also record data and export it for plotting of graphs.
Are you speeds varying across the day, e.g. fast around breakfast but slow in the evenings?