Macbook Pro doesn't wake up from sleep, RAM slot empty after force restart
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.