Hey, welcome to TechIMO, dfrav!!
Well, first things first: is it hardware or software related? Easist way to check the cable is to swap it to the PC that works fine. Use the same router port and different cable and see if the PC that is normally fine plays up. If it does, get a new Cat5E rated cable. Simple.
Then check to see if a different router port also causes problems. If it doesn't do anything bad on the other PC on it's usual port, but plays up on the one your PC is normally on, then your router needs fixing or replacing. Ports can go bad sometimes.
If cable / port work fine with the other PC, might be software settings on your PC. Another thing that can cause weird connection issues are viruses and trojans, so do a good, thorough virus scan (online one at
www.symantec.com or something is fine).
Hope these ideas help to check out what it might / might not be. Let us know how it goes!!
Cheers
Mick