How can I tell what caused my PC to shutdown?

I've been having sleep problems with my Media Center PC I built a few weeks ago. It will go into sleep on it's own or if I manually put it to sleep but just about every day to day and a half, while in sleep mode, it just shuts down.

I've run 7 different tests and all come back perfect...ram, hard drives, video card, peripherals, there are no temp problems that I can find....I've checked everything.
I've installed the pro version of WhoCrashed and it shows no reports of crashes and there is no minidumps. I'm not sure how I can tell what caused this to happen.

I've installed Passmark Sleeper which shows my system supports S3 & S4 sleep, found no problems in the UEFI settings and still, this happens. Since I use this as a DVR with Media Center and a Ceton cablecard tuner, I don't want it to run all day unless it's recording. Away mode is just not 'off' enough.

I'm not sure how to check this out and appreciate any suggestions.

Thanks

 

Thanks for the reply.
Not sure I understand. I have hibernation set to off but it's an option on the start button. I never use it. It's set as 'never' in Power Options. So, are you suggesting I disable it all together with the powercfg command?

I have an almost identical system that runs perfectly though it's not used nearly as much. But, the hardware and software is almost a twin.

 

I left the machine on all day and set to not go to sleep and as I thought, it stayed on. So, something about sleep mode causes the computer to shut down.