MY Xbox live MTU.....?

my 360 keeps failing MTU test, its wierd tho because it will work fine one day, then the next day itl ul just start failing? can anybody tell me whats wrong and what i kan do about this? I went to my router settings and couldnt find anything to do with MTU(I have a belkin) My 360 is bridged to my laptop by the way to aquire internet signal..