Cannot connect to XBOX Live!?

Am trying to get xbox live working, Says my mtu's aren't high enough but have already checked and they are. Xbox tech support cant help me they have just pawned me off onto my ISP and they have sent me to my Modem provider Netgear and the have sent me back to my ISP! Have tried wireless and cable straight from modem. Sometimes it does connect but stops when it starts updating. Any help would be awesome Thanks