Do you happen to have an Nvidia motherboard? Apparently BitDefender can cause such internet slowdowns and stops. I've seen BD Internet Security mentioned several times in relation to such problems. The way it interacts with Nvidia boards (and possibly others) doesn't seem to jive the way it's supposed to. My suggestion would be to temporarily uninstall BitDefender, restart, test the connection, if it doesn't work, do the fixing procedure, test the connection again, then restart and check one last time to see if the connection works after the restart. If it is still failing after BD is uninstalled and the fix has been done and computer restarted, we can probably rule out that possiblity.