The last few days streaming video on Youtube and any number of other online sources is suddenly unwatchable on my iMac due to constant rebuffering. Is this likely to be an issue with my broadband provider (Sky) or could there be a problem with my Mac? I'm running Lion 10.7.3 with the Chrome browser. This morning my download speed was measured at 4.09Mb. I am getting similar results using the Mac Mini I run on the same network.