Using http://www.thinkbroadband.com/speedtest.html will let others see the graph of speeds which will help understand what you are seeing
What model is your broadband router and powerline plugs?
It is rare but using a different port on the router may help
my router is the newest one from sky and the model number is SR102.
My powerlinee is tp-link av500 and the model is pa4030kut.
i have tested my speed on this site and it comes up fine and says im getting 30+mbps but on speedtest.net its still saying its really slow and when trying to download something it is also slow. i tried to download a 50mb test file which should take around 14 seconds but its taking 7 minutes.
i have also tried different routers of the same model and its the same.