Slow Internet - Reset To Windows Default Settings Feature Which Does Not Work?

So just recently my Internet has been running a lot slower than normal at different times of the day (completely random, could be fine for hours then just turns slow). I know it's not the Internet itself, since it's only happening on my computer, and none others in the house - and I'm far from capped. I've run AVG, Spybot & Malwarebytes' scans and have found no infections of any kind. I've run TCP Optimizer and used the "reset to windows default settings" feature, which also hasn't worked one bit. I've tried different Ethernet cables and different plugs in the modem and no improvement.