When I do swat, everybody starts running into the wall
Does mumble crash at the same time?
If the games run properly in singler player, it's pretty obvious that the problem is related to the internet connection.
Hardware failure could include LAN-card, cabel between PC and modem/router, modem/router, cable between modem/router and wall, or as already said a provider issue (cable behind the wall ).
It could also be a software issue, maybe the operating system or a firewall. Did you installed new software lately (before the problem appeared)? Could also be some virus, worm, trojan, rootkit, malware shit. Or a new driver with messes things up.