My download speeds are decreasing for some reason. When i restart my PC and start downloading something it will stay in the 250 kbs range, but after a few minutes it starts decreasing until it hits rock bottom and starts fluctuating around 20-80 kbs. my internet speed is also 100 Mbps and im plugged in directly to my modem.
To get a good rating on your download speed go to www.speedtest.net and post the results here. You very likely don't have a 100Mbit connection, that is mostly likely just the speed you are connected to your modem at. You should be able to lookup what speed you pay for from your ISP's website.
The fastest offering where i'm at is 50Mbit which is $100 a month just for reference, and that's in a fairly developed area.
As boone145 mentioned rebooting the modem is always a great first troubleshooting step.