Spontaneous Shutdown/Reboot

Recently (within the last couple of months), I have been having sporadic,
spontaneous shutdowns or reboots. The PC is only about 18 months old, and I
have made no changes to it in the last year or so. When it shuts down, I can
restart it without waiting; it starts up fine and immediately.
There are no entries in the event log other than for unexpected shutdown.
Core Temp shows no unusually high temps; it has never logged anything closer
than 35°C to TjMax.
The inside and vents are clean- no cat hair or dust. The fans are all clean
and running well.
CHKDSK shows nothing wrong.
MEMTEST86+ returns no problems.

My specs a
Lenovo H50-55
AMD A10-7800 Radeon R7 (12 Compute Cores 4C+8G) 3.50 GHz
12GB Ram
2Tb C: drive (came with the PC)
1Tb D: drive (used for data and movies, etc.)
SATA DVD drive
Nvidia GeForce GTX 750 video card
Corsair CX Series 600 Watt 80 Plus Bronze Certified Modular Power Supply
(replaced the original 180W PSU to power the video card- wasn't impressed
with the GPU graphics)
Logitech M705 mouse
MS SideWinder X4 keyboard

Windows 10 Home x64 V.1709 (Build 16299.125)

I'm getting ready to tear it down and check for puffed-up caps on the MB and
PSU, give it an extreme cleaning, and pull the heatsink and apply new paste
(replaced it about a year ago). Will reseat all cards, drive connectors, and
memory sticks. Might as well replace the CMOS battery while I have it apart.

Anything else I can check while I have it apart?

Thanks for any suggestions :-)