Youtube not working?

Hi all,

Have just got hold of a new imac (late 2012) and initially youtube was running fine, but now just will not play videos, or takes a long time to do so, the internet connection here is very fast (20mb+) so its not that, and playing video from anywhere else is absolutely fine, just wont seem to work with youtube, and have tried on both chrome and safari, any ideas!?

 

I'm almost sure of isn't that as my pc will load them straight away. It's not so much loading time but more so that flash seems dodgy, like if I run the cursor along the base of the video it shows the preview and any ads / annotations load straight away! But the screen just remains black, sometimes plays, most of the time doesn't!