Hi all,
Hoping for a little bit of guidance in troubleshooting what I suspect to be a dodgy GPU.
Over the last 4-5 months I have been experiencing seemingly random crashes while gaming. About half the time I get a black screen for a few seconds before the computer restarts. The event viewer simply states "The previous system shutdown at --:--:--  on ‎--/--/‎---- was unexpected." The other case is where only the application crashes and I get an error message along the lines of "graphics runtime error" where the event viewer says "Display driver nvlddmkm stopped responding and has successfully recovered.". 
Temperatures appear to be normal and generally hover around the 60-75c mark when under load, and they don't appear to spike when crashes occur according to Speedfan temperature logs. This is the same for all other components. Sometimes I can go 8 hours straight without a hitch and a sometimes it'll crash 2 minutes into a session. I generally use my computer to play Destiny 2, but I have experienced the same problems in the Halo collection too.
 
OS: Windows 10 Education 64-bit
MOBO: Gigabyte Z170X-Gaming 7
CPU: Intel(R) Core(TM) i7-6700K CPU @ 4.00GHz (8 CPUs), ~4.0GHz
Memory: Mushkin Redline Ridgeback G2 16GB
Storage: Mushkin Atlas Vital MKNSSDAV500GB-D8 M.2 2280 500GB
GPU: EVGA GeForce GTX980TI Superclocked+ ACX2.0+ 6GB
PSU: EVGA SuperNova 750 G2

Happy to provide any additional info that may help. Many thanks for your assistance in advance!
Cheers

Sounds like solder cracking somewhere on the PCB may have occurred..That was not uncommon with cards of that era and can cause issues such as artifacting, freezing, restarts, BSOD, more.
 
If all else fails, you can disassemble and bake only the PCB (aka the 'oven trick') to reflow any solder cracking that might exist, then hope for the best.
 
If you bake it, do it only with the GPU chip side up and not down.
 
Baking may only fix it for a short time, or a long time, or not at all if there's a component failure.
 
Disassemble the card..Preheat oven to 200C..Place the PCB with the GPU side up on a cookie sheet/baking pan..Bake for 10 - 15 minutes..Lots of other videos on YouTube about how to do it.

Yikes... Sounds like something I would only want to attempt as a last resort. Is there anything else you could suggest to try and narrow down the fault before I take a shot in the dark on something like that?
Appreciate your response.

You could downclock the GPU frequency as far as possible (probably about -105 MHz or so), or add more GPU Core Voltage using EVGA Precision or MSI Afterburner and see if it helps any.

Thanks, I'll give that a go. At this point is it safe to assume that it's the GPU that's the problem?

Comraade
At this point is it safe to assume that it's the GPU that's the problem?

IMO, almost certainly..I have an old EVGA 9800 GTX that did the exact same thing as your 980 Ti..I baked the thing about 5 times within about a year..Every time I baked it, it worked again for a while..Still have it, still works..It's plagued with inferior solder.

I'm running 980 Ti's in SLI and I get this crash to unless I down clock as far as I can, -90MHz on core and -201MHz on memory.

Your issues sound similar to what I've been dealing with on my 980ti Hybrid over the past couple of months ( since last update, funnily enough - tin foil hat lol). It would be fine whilst on light duties, but anything graphic intensive would result in BSOD, Reboot or PC shutting down. Finally, today, it let out the magic smoke whilst powering up. Tore down the card for a quick inspection and found a blown out driver.  The others show signs of heat on the back of the PCB. I don't think any 'oven bake' method would have saved this one.  I'd be interested to find a reason for this failure as a quick google brings up couple of other examples with exactly the same failure mode. I can't speak for the other examples, but I can tell you my card led a pampered life; no mining, no over volting and no long period of stress.  Input welcomed. 
 
 

I have noted that several elite video cards from that period were caught up in the change of solder to a lead free variant
 
The affected hardware galore including hard disks, motherboards and peripherals generally.
 
It took time to figure out how to make solders that were more reliable and not short out or fail over accelerated testing etc
 
 

that really sucks!