Inspiron Internet Issues (Windows 8)

Hi, I have an Inspiron 660 desktop running Windows 8 64bit and for the past 2 weeks the internet has been really slow. Speed tests show much slower download and upload speeds and a much higher ping. As a result I am unable to stream audio, video, or games. The Dell is connected to my Vizio XWR100 router via Ethernet cable. If I connect via WiFi the speed remains slow. However, if and only if I turn off the Inspiron, then the other computers and phones that connect to my router via WiFi have normal speeds and low ping. So the problem appears to be caused by the Windows 8 PC. Here is a screenshot of the device manager:

I've tried to fix the issue using several methods:

• Restarting the router and modem countless times
• Changing router settings
• Updating network card drivers automatically using device manager
• Installing network card drivers manually by using the Dell support website
• Uninstalling RealTek PCIe GBE Family Controller and restarting PC so Windows installs it again
• Disabling IPv6
• Disabling the Ethernet connection
• Changing the DNS server
• Flushing DNS via command prompt
• Used Windows Troubleshoot

The Vizio router and the Motorola modem seem to be working fine and have the latest firmware. Does anyone know how to fix this? Every time I restart the router the speed is normal for about a minute and then becomes and stays slow unless I restart it again or turn off the Dell.