Could symantec cause multiplayer games to freeze but not singleplayer games

yes but no...symantec would probably allow the game to run in single player but when you go into multiplayer and other computers try to receive connections, it could block the connection. usually when this happens it creates a popup which asks if you want to allow connections to and from the game which would minimize your game, which would greatly slow down your computer, and possibly cause it to freeze. and that's exactly why i don't use symantec...if you want to get around it, you need to find out which ports the game uses and manually configure it to allow data transfer in and out of those ports.