Some youtube videos don't load.

Recently I have been seeing some videos on youtube with a playback 'Speed' option (0.25x, 0.5x Normal 1.5x 2.0x). Videos that have this option don't load. They little round thing continuously turns and nothing happens. The same videos however work on Google Chrome so i know it is a problem with Safari. This problem only seems to happen at school where we need to use a proxy to access internet, beacuse at home I never see videos with a 'Speed' option. My freind has a mac and uses Safari and he says he never sees them either, even at school.

Thnaks for your help.