pixillated playback

[brendanxx]

I have 2.2.6 umbrella 64 bit on my macbook el capitan. I'm experiencing pixillated playback (most noticeable on faces which suddenly look ancient then back to normal, but also making skies, any blocks of colour, squadgy (the best way I can think of describing it) and pixillated.
Not really a computer whizz and would welcome any advice as in the past VLC has worked perfectly.
Should upgrade to Sierra and hope the problem goes away?
Tried KMplayer which gave a great picture but often very jerky playback - so really want to get VLC OK again.
Thanks
Brendan