Plug an ethernet cable into this computer ????

Hello guys , i have a problem with my desktop pc and I really can't find a solution to it. Recently , I started to get dc and the only way of making it work again was to turn off my modem, wait 10 seconds and then restart the pc. Recently i was downloading a game from steam and after 3/25 gb it dc again. Troubleshooter reported 2 problems. The first with an red x stating that an ethernet cable is not properly plugged in or it might be broken (my cable is new and it works fine) and one with a yellow triangle stating that local area connection doesn't have a valid ip configuration. Ok so I gave up and did 2 things: first i checked my hardware. They said it works perfectly . Then I unplugged my modem thinking that this was the problem. I plugged the cable directly into my pc and had to use dial-up connection. Ok , i entered the user and pass , connected and after a few days of perfect connection i've decided to resume the download. After downloading another gb it crashed again . So out of frustration i took my pc to my friend and used his connection for the download . It worked perfectly . Now i am back home and guess what? After trying to download a large file it crashed again and now it won't even connect back. i. Have never seen something like this , it just collapses if it uses up more that 3-4 mb/s . Now the troubleshooter says the swme about the cable and the ip . My settings are to obtain an automatic ip ... My driver is fine , my cable is fine , wireless on laptop works , so it's not the provider ... My brain is bleeding .Any ideas About this?