hello and
welcome I am not sure if its been suggested yet ... but are you running these speedtests over Wi-Fi..? or over a LAN Ethernet cable connected to the computer? - just Wi-Fi can play havoc with high speed downloads.
another thing is that some testing servers are unreliable and not all servers are capable of testing high speed connections, also some servers may be busier than others.. and its also possible at the time of the lower speed test your ISP may have had some intermittent congestion which would affect the results, its probably something I've just mentioned rather than a fault causing speed to be fast one minute on testing and then a bit slower the next.
the best thing I find is to try various testing servers on
speedtest.net until you find one thats reliable and reports good speeds and can be verified against other reliable servers and testing websites. A good speedtest is one you can run on your PC as a program that downloads files and times how long it took, I use
JD Auto Speed Tester which always gives me true reliable results, you can change the download and upload test type, files and locations too - it defaults to a multi-threaded test for a set duration (10 seconds downloading 3 x 100Mb files from 3 locations) but one can change it to fixed file size test too where it will download one file from one location, there are many locations and file sizes to choose from but I think the default multi-threaded 'duration' test is the best one, you can add & remove to that too.
JD can also be set to run silently at boot and auto test every X minutes so you can see how your speeds are throughout a set period.
anyway.... its always best having an unlocked modem so you can pull stats from it if you ever have problems, if you thought router stats lite was great.... wait until you see Eric's (Roseway) RS-W program... theres pretty much nothing it doesn't do haha
by the time we are finished with you , you'll be an expert broadband technician!
screenshot of JD after 3 tests run: