Youtube won't buffer please help?

Hi I've been watching Yotube for a while now and I was just fine until recently I just couldnt play videos anymore without start stop. I've cleared my cache and history. Nothing. I've added &fmt=18 at the end of videos url. Nothing. I've cleared disk space on my Comp. Nothing. I've defragmented my hard drive. Nothing. I have change the quality of videos. Nothing, I can only watch in 240p which is almost unwatchable.I've gone from Mozilla 3.6 to Internet Explorer to Google Chrome. Still nothing. What can I do to stop having this stop/start/stop/start problem when I play videos. It's not even just YouTube either, it's any site I watch videos on. Please help. Thanks.