IMac – 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.