What type of connection do you have? If you have cable you are sharing a pool of bandwith along with everyone else in your block( not a street block, but a block of customers to the isp). The after 8pm drop might indicate more people are on during this time.
Also what activity are you doing at...
I have noticed recently that when I pour coca-cola ( from a regular red can ) into a plastic cup then mix crown royal in it that this produces "eruptions", small streams of liquid shooting up from the surface of the drink. What is causing these? I have mixed drinks many times and never noticed...