Speedtest requires flash. Just saying.Do you have more than one system on the internet?
Running speedtest on more than one would be the best way:
http://www.speedtest.net/
Some of it depends on what type of service you have. If DSL, you usually have a dedicated line to the ISP. If on cable, you will be sharing bandwidth with all others on your segment. There are times of day when things slow down - just like rush hour on the freeway.