Strange Imac sleep/wake problem

I have a 2011 Imac running Yosemite with a strange sleep problem. If I let the the Imac go to sleep on its own, there is no problem in it waking. But if I put it to sleep, about 40 % of the time it will not awaken. This problem may have started with Mavericks, but it was not a problem with Lion and Mountain Lion.

Any ideas.

Thanks

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