Is it always 20Mbps. Sometimes when I'm on the internet the speed drops to 5 Kbps and I have a 1Mbps ADSL line.
And remember that we have a 5.12 Terabits/ps line under the water connecting us with the rest of the world.
Also, when high paying customers need bandwidth, they'll just reroute yours to them.
Your processor might be damaged from overclocking or age.
Another proposition might be that Windows is giving problems.
Maybe get a second opinion since I am only fifteen
Happy New Year and hope this helps.