Okay, two questions:
First, have you tried running the Speedtest from a different server?
Secondly, is it happening in the real world or only when you run this Speedtest? What I mean is, if you try to simply download a large file, or play a stream, are you seeing the same pattern?
One thing I like to do, as a real world test, is to download a real file from a real web site. Star Trek fans are a busy thriving lot, and they have video fiction galore all over the place. One example is the Star Trek: Farragut series. And one of their download sites is very well maintained and very reliable, located at »farragut.novelhost.net/
So, whenever I'm seeing something strange, and I don't know whether it really is my connection or something anomalous with my speedtest, I go to that site and I start downloading one of their largest files. I usually go for one of the gigantic .iso files.
I let the download go for about 5 minutes or so, because I notice the download speed takes a few minutes to stabilize. Then I just watch it for a minute and see if 1) it's consistent and 2) what the speed is. Nine times out of ten, I discover everything's fine and my fancy-schmancy speed test was acting up.
One reminder: Many file download data clocks use KiloBytes and MegaBytes instead of kilobits and megabits. So don't forget to multiply by 8 to convert it to speed figures that will make sense.