New 5K iMac shutting down

Hey everyone,
Almost a year ago I took the plunge and thanks to E.J.'s advice, upgraded to a fully maxxed out 27" 5K iMac with SSD and aftermarket memory. It's currently running Sierra, as I haven't wanted to upgrade to High Sierra since I use Nik Software a lot. I've been super happy with it (it blows my old MacBook Pro out of the water), with the exception of one thing. It will frequently (multiple times per week, sometimes a couple times per day) freeze and then shut down. I'm usually not doing anything extremely intensive, such as working on a massive panorama in Photoshop, just basic single image edits in Capture One Pro or small tweaks in a small project in Final Cut Pro. I only have photography and video editing programs and internet browsers installed ont it. I've noticed it the most in the following scenarios:
- Working in Capture One Pro 10 with Adobe Bridge open in another virtual desktop, editing a photo that's saved on a 5TB external drive. I'll adjust a slider, and all of a sudden the screen will either briefly freeze or go black and then the computer will reboot. 
- Adjusting sliders in the video inspector panel of Final Cut Pro. The computer will freeze for maybe 15 seconds then reboot. 

My questions are:
- What could be causing this? I did a Google search and discovered that some iMac's apparently don't play well with aftermarket memory, although I believe all the chips in question came from Crucial and I got mine from OWC. Is it related to working off an external hard drive?

- Is there some kind of diagnostic or monitoring program I can download that will somehow help me figure out which component is causing the crash? I don't even get an option to send any kind of error report when the computer turns back on (not that I understand anything in those anyway).

Thanks in advance,
Nathaniel

Hmm, I would definitely say there is something wrong with a hardware component somewhere - likely either graphics or memory. If you take out the aftermarket memory does it still crash? If so, then take it back to Apple and have them fix it. If not, then the culprit is the memory.

You can run built-in test which is pretty much useless;
https://support.apple.com/en-us/HT202731; and it won't tell you much.
Taking it to Apple and expecting fix in those kind of situations is 50-50 chance at best.

What you can do is try to create new account and try to replicate the problem. If you can
replicate crashing in new account then try swapping memory back to original.If computer seem to be working fine while you logged in into new account then some of your preference files is corrupted.

Also you could try fresh clean OS on external drive and see if you can replicate crashing.

Maybe try a NV/PRAM and especially a SCM reset. Good luck. Joerg

https://support.apple.com/en-us/HT201295
https://support.apple.com/en-us/HT204063

Thanks for the suggestions so far. I've just replaced the memory with the original stock sticks, and haven't had a crash yet while doing the same thing that got me a couple crashes this morning. I'll probably have to wait a couple days to be sure, since thanks to Murphy's Law I probably won't get a crash now that I want one. I should have done the built in test while I had the memory still in, but I'll swap it back out in a few days and see if that comes up with anything. If it does seem to be the memory, is there a supplier that sells the same exact memory that Apple uses? I'd really rather not pay the prices Apple is asking for on their website.

On a related note, is there anything that can be damaged from repeated crashes (specifically external hard drives)?

Thanks again,
Nathaniel

nchild wrote: If it does seem to be the memory, is there a supplier that sells the same exact memory that Apple uses? I'd really rather not pay the prices Apple is asking for on their website.

On a related note, is there anything that can be damaged from repeated crashes (specifically external hard drives)?

Apple is using Micron's memory in 2017 iMacs I believe. Crucial is also re-selling same chips.
You can remove chip and check the number. It really doesn't matter which manufacturer you use.
Sometimes you just gonna get bad chip.

Repeated crashes may sometimes cause damage to directory tree. In theory it never should happen but
from my experience it happens more than not. On top of that if you have 10.13 and apfs there's no tools
available to fix damage to directory and the only solution is to reformat and restore. Disk Warrior wasn't updated yet to
fix apfs volumes.