MTU SETTINGS FOR XBOX LIVE?

yesterdayxbox live today. i couldnt connect to xbox live. it said the mtu needs to be higher then 3641 i think. i went to my router page and its at 1500. i tried it again. i unpluged everything for a 1 minutes and plugged back in and it worked for about 10 minutes and i got disconnected again. and now it wont work..... anything?