Purchased refurbished Dell Optiplex 755 SFF came with Win7.
Performed update to Win10.
I only use this machine for watching tv in a spare room, so no important data.
No extra software / bloatware / antivirus / etc is installed on it.
Only USB device is an IR receiver for remote control.

Put PC into SLEEP mode (S3 in BIOS), come back later and wake it up, screen comes back, I can use remote to navigate (or just leave it alone), then in less than one minute the PC completely (without warning or blue screen or anything) shuts off and restarts. Happens regardless of whether the tv software is running or not running.

Here is what I have done so far:
-Verify all devices in device manager are good.
-sfc scannow (found nothing).
-ram test in BIOS (found nothing).
-disable Hybrid Sleep windows power advanced.
-reseat ram into mobo slots.
-BIOS update from v19 to v21.
-GPU drivers from whql to latest beta.
-tweaking various USB power settings in device manager.
-tweaking power settings for network adapter in device manager.

note: I have an almost identical machine (optiplex 760) in another room and it is NOT having this wake from sleep issue.

Anyone know how to check logs to see why it's rebooting like this?
Maybe try pulling ram sticks and testing sleep?
Anyone think trying a clean install would help or is this hardware?