Late 2015 iMac 27 freezes than crashes and restarts - often and apparently randomly I purchased this late 2015 iMac 27 with built in 8GB of memory in September of 2016.  Shortly thereafter I replaced the original memory with Adamanta 32GB (4x8GB) Apple Memory Upgrade for Late 2015 iMac 27" DDR3L 1867Mhz PC3-14900 SODIMM 2Rx8 CL13 1.35v RAM from Amazon.  I also cloned my internal drive to an external SSD drive, and use that as the startup disk.

Everything worked fine along with significantly better performance.  I did however, start experiencing freezes, and after 20 seconds or so, a reboot.  These appear to be random, although mostly while using a web browser - usually Safari, and they continue to this day.  I may go a week or so without a crash, but sometimes I get several in one day.

Not sure if this would have happened without the third party additions as they were done about a week after purchase.  I suppose I could reinstall the original memory, and possibly clone back to the internal drive and see what happens.  But the computer is still under warranty, and I purchased Applecare+ (something I wouldn't have done in the past, but my own experience with a MacBook Pro and friends' experiences with Apple products, it seems they're not what they used to be) so I figured I'd take it straight to the Apple Store.  I saw this site and decided to give it a try before I do that though.

I'm running under El Capitan version 10.11.6.

Thanks for any ideas or suggestions.
You used a different type of RAM than specified by Apple. It's a small difference, but it is possible that it caused the effect you are describing. You purchased this RAM:


  DDR3L 1867Mhz PC3-14900 SODIMM 2Rx8 CL13 1.35v RAM


Apple's site specifies this RAM:


  PC3-14900 Unbuffered Nonparity 204-pin 1867MHz DDR3 SDRAM


What's the difference?
It's DDR3 vs DDR3L.
The "L" stands for low powered. Standard Apple RAM, and replacement RAM from sites such as OWC, operate at 1.5V. Your DDR3L RAM operates on 1.35V.