Alienware Aurora R4 GTX980 Sudden Slow System and Unplayable Gaming FPS. Please help!!!

Hello! First off, thank you so much for taking the time to read this and for any help you might be able to provide!

I purchased my Alienware Aurora R4 a little over 2 years ago and have absolutely loved it as well as the customer service I have received from AW. I am a heavy gamer and enjoy games such as The Witcher 3: Wild Hunt, Fallout 4, Ark: Survival Evolved and recently No Man's Sky (to name a few). I have had very little issues with my system until the past couple months.

A couple months ago I randomly started experiencing Frames Per Second (FPS) drops, games crashing and overall unplayable conditions. After a long 'remote assistance' session from AW, they deduced that the issue stemmed from my graphics card (at the time a GTX780) and sent a technician to replace it. For the trouble and inconvenience the replacement consisted of an upgrade to the GTX980 which also ran flawlessly on max settings of all above games and beyond.

Here's where it gets good. 2 nights ago, mid game I dropped from around 100FPS to 5 or 10. Restarting the game or switching games yielded the same low FPS. Then I started to notice my system was bogged down entirely. Slow scrolling on web pages, google struggling to open a single image, etc. A fresh restart of the computer seemed to fix this, until about 5 minutes of playing a game where it would do it again and has continued with the low FPS.

I spent hours searching the internet with more computer savvy friends trying different approaches such as: disabling superfetch/prefetch and windows search/notifications, deactivating Steam overlay during games, malware/adware/virus scans/clean-ups, memory test (came out fine), page file size increase, reverting to previous driver versions (and back again) as well as switching video card slots to ensure it wasn't just that. Nothing helped.

Finally 2 calls to AW and hours of 'remote assistance' essentially trying the things I had already tried deduced, again, that my graphics card was bad and needed replaced so they scheduled a technician to come do that again next week. HOWEVER! After I got off the phone we took my GTX980 out and put it in a different machine, ran flawlessly all night. We also put a GTX960 from a buddy's computer and my machine yet again bogged down. I don't know if I'm supposed to do that and don't want to void my warranty so I haven't mentioned that yet...

Nonetheless, I am lost. My computer troubleshooting skills are maxed out and I would love any assistance or thoughts you all might have! 

My Rig:

GPU: GeForce GTX 980

CPU: Intel(R) Core(TM) i7-4820K CPU @ 3.70GHz

Memory: 16.00 GB RAM

Nvidia Driver Version: 372.54

Operating System: Microsoft Windows 10 Home

Thank you again so much for any help!