what other sites? and are we talking about the same video length? baka naman sa ibang sites pinapanood mo is mga tipong 3min lang tapos sa youtube yung mga tipong tig 1 hour na video. sa speed ng net mo mag buffer talaga yan pag mataas ang video length, 1.67mbps only translates to about 200KB/s
anyway, try to clear your cache and remove youtube cookies. see if that helps. though if your problem is only buffering and you are not seeing an error, it probably wont help