My mid-2009 Macbook Pro, running Mountain Lion, very often does not wake up from sleep. When I open the lid, there is a black screen, but the fans are still running. My only option is to force reboot by holding down the power button. When It finishes rebooting, only half of the installed RAM is recognized - System Information shows that Bank 1 is empty when there is actually a stick of RAM installed. I have tried swapping out the RAM, booting from other disks, reinstalling, and searching for evidence in the log files, but the problem persists. The Apple Genius Bar didn't even know what was wrong with my computer. I am at a loss - what could be the problem?
If you've tried reinstalling and resetting PRAM / SMC etc and it's still occurring on a clean install then that rules out a software issue. If you've tried another ram stick that rules out faulty ram.
Sorry to tell you, but it sounds like a motherboard issue, which means $$$. That's the only other thing I realistically think could be causing the ram fault.