"I have a network card that functions fine with light use, but quits functioning after heavy use. I replaced it with a brand new one and still have the same issue, also updated drivers. It is a wired D-Link card."
provavelmente não é a placa de rede.
"The Internet seems fine for a small amount of web browsing but when I run a bandwidth test it starts out fast and slows quickly until the card completely quits; I have a constant ping of the gateway going while I run this and it starts timing out after a couple seconds into the speed test"
Pode ser o gateway - a menos que outros sistemas funcionem bem.