External monitor goes black for short periods on Yosemite MBP mid 2010

Can anyone advise on how to stop this happening.

I upgraded to Yosemite 10.10.1 on my mid 2010 MacBookPro 17" and recently, the external monitor attached to it goes black for a few seconds but then comes back to show the Desktop etc.

When I boot into WIndows 7 64 bit Pro (via Boot Camp) this never happens. So I don't think it's a hardware issue.

It also never happened with Mavericks.

It seems to have started recently in the last month, but I upgraded to Yosemite 3 months ago and didn't see it then.

Please advise.