MacBook Pro will not wake up from sleep

My MacBook Prof laptop will not awaken from sleep sometimes. The laptop could be left open and I walk away for awhile. Come back and it is in the black screen status. When I hit a key to wake it up....it won't. Have to hold the power key down for several seconds to turn it off, then push power key to turn back on. Also, sometimes I close the lid and walk away. Come back several hours later and it will not wake up. Went to the Apple store a few weeks ago. They ran a quick diagnostic on my hardware and said it was all fine. They suggested they wipe clean the hard drive and I start fresh. We did and everything was fine for a few weeks. Now it starting all over again? Very frustrating and time consuming to have to restart your computer multiple times a day.

Any suggestions or thoughts? Could it be a hardware issue? Should I let the Apple store run the 7 hours hardware test?

Thanks.