El Capitan - Display corrupted after sleep

I have a 2014 Mac Mini running El Capitan. A couple of times the display has been corrupted when my Mac wakes from sleep. I used to see this happening with Yosemite occasionally when I used a mini display port to VGA adapter, but it stopped happening when I switched to using an HDMI cable. Now it seems to be happening again with El Capitan. However, it's slightly different with Yosemite, the display was a jumble of colours whereas with El Capital it's like a deep, pink haze over the display (pretty, but not very practical!). Restarting the Mac fixes it. It seems more likely to happen when the Mac has been sleeping for some time. Has anyone else seen this problem? Thanks.