do you use a wireless router? if so, make sure you have it password protected, someone could be close by downloading stuff, IE: music. Also, try reseting your gear. Start by shutting all pc's down. Then unhook the router first and leave it be, then unhook the modem, wait 10 seconds, reconnect modem, then reconnect the router. If you have a switch, then shut that down before the routers and hook back up after the router is turned on.
MODEM --> Router --> Switch