I had that. Right click on the youtube video. Then click settings. Then untick "enable hardware acceleration". Last refresh the video.
That did it for me.
i5 4440 3.1Ghz, Asus H87M-E motherboard, 8GB DDR3, 2GB DDR5 nVidia card, 2x500GB Seagate hd, Coolermaster 750W PSU running Mint 17.1 MATE and Win7
HP mini, 1.66 Ghz Intel atom processor, 2 GB DDR3, 320 GB hd, Mint 17 MATE