Optiplex 980 network card configuration problem

Hello everybody,

I bought a PC recently and wanted to have 1gbps as my networking card can do it. But if I choose 1gbps for my Win 10 64 bit my network card is disabled. Everything is fine with 100mbps. I thought I could remove the limitation in BIOS but could not find anything about my this limitation. Please help. I have 500 mbps but max which I can get is 45mbps.  It was the same problem with Win 7 32bit.