Internet slow on only one computer (wired)

My ISP is Wave Broadband. I run my coaxial cable into Wave's modem and then my ethernet cables out from there to 2 other wired locations in the house. In the upstairs wired computer, i consistently get 99 mbps download speeds, but for some reason, on the desktop downstairs, I occassionally only get 4-6 mbps. if I reboot the computer, it then fixes itself and gets the blazing speeds again. Even if I leave the computer idle, but connected, the speed eventually gets degraded back down to 4-6 mbps. I never get something in between.

I had Wave come out and connect to that cable and they got 99 on their laptop and when I connect my laptop, I also always get 99, so it is definitely my desktop computer. Icannot figure out why a simple reboot fixes it, but then it degrades, obviously triggered by something. We fully ran virus scans and cleanups and updated all the drivers. I do not have AMD Quickstream and there is no router involved except the ISP's and we have eliminated that as a probem sinceother computers have been tested on it on various ports and all work well. This was not a problem for a year until about 3-4 weeks ago.

It is a 1 year old Dell Windows 7 and runs great, so I am a bit confused why this would happen so drastically and is always remedied temporarily by a reboot. Thoughts?