Windows 8.1 Ignores power settings and hibernates after 5 minutes


New Member
My PC ignores all power settings and hibernates after approx 5 minutes of user inactivity. It is a Dell Vostro 200, which I had been running successfully on Windows 8 Pro. The problem appears to have been the result of upgrading to Windows 8.1.

Regardless of the power scheme settings, the PC goes into hibernation after 5 minutes of "inactivity"; i.e. if there is no user keyboard or mouse input. The hibernation also ignores most background processes, so it will go into hibernation even if I am streaming internet radio. I use Windows Home Server, and the daily backup process is currently failing because the PC wakes up to start the backup, but hibernates after 5 minutes, in the middle of the backup process.
The only exception I can find is YouTube - hibernation does not occur if I watch a video longer than 5 minutes, even if there is no user input.

I have tried various suggestions from other forums, without success:
The video drivers up to date.
BIOS is up to date, and ACPI is set to S3.
I have run power settings troubleshooter which reset power settings to "Balanced".
I have tried setting power to "Always on"
I have tried a clean boot process, disabling all start ups and other services, but the problem still occurs

As noted above, the PC was previously working fine, and the only change I can identify is applying the Windows 8.1 upgrade.
Does anyone have any ideas how I can resolve this, or any suggestions for debugging the cause, please?

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