Cannot maintain internet connection beyond 10 minutes.

I have Charter cabel internet and am running a wireless network at home.  I have ethernet lines to router, and back to my primary desktop PC.  Those PC's (laptop and another desktop) with wireless internet have no issues.  They stay connected as you would expect.

My primary PC drops connection after 10 minutes.  I have run virus/spyware/a host of diagnostices, and no issues.  I tried connecting direct to my PC from my cabel modem and have same problem.

How do I check my PC's ethernet connection to see if  it is bad?  Or is there something else I shuld be looking for.

Very frustrating...hope you can help.