17 R4, issues, help and guesses would be appreciated

Hi all. I just bought a used Alienware 17R4 with a core i7-7820HK @ 2.9Ghz & a geforce GTX 1080. The internal display is the 1080P IPS 60Hz model. Before I purchased the laptop, the previous owner had just run alien respawn on the thing. When I got it, the system was at the sysprep stage, meaning I was greeted by the standard Windows 10 setup screen (sign into your account, set up keyboard, region, lots of chat from Cortana, etc...) when I turned it on. So I get my new laptop home, excited to try it and the first thing I do is connect up my external monitors. I want to try a 3 monitor setup! One of the monitors is 4k (connected through displayport), the other 1080p (via HDMI) - both @ 60Hz; the third monitor would be the laptop's internal display. Immediately, I noticed an issue with the 4k Panel. It was so laggy! Although windows reported it was running at 60Hz, it looked more like it was running at 30Hz. If I tried to play youtube through it, 50% of the frames would be dropped. The 1080p monitor was working fine. Here's the strange thing - the 4k Monitor would work normally if I disabled the laptop internal display! I thought it might be a driver issue, so I updated windows, graphic drivers (both GPU's), patched the BIOS, got the latest MB drivers from Dell, etc. This did no good at all. At first, I disconnected the 4k display, and things were relatively normal with the internal display & external 1080p monitor. BUT... I think I'm getting far worse performance than I should be - most games struggle to maintain 60fps at 1080p.... Also, I definitely meet the recommended specs for Davinci resolve, but the timeline scrubbing performance is garbage! This aside, I still wanted to use my 4k panel. I read online that to support a 4k monitor you need to use FN+F7 to set the machine to discrete graphic only mode. I did this, and while it does resolve the issue with the 4k display - it runs fine in this mode - after using it for 20 minutes, all my games stopped working normally. I could only get 20fps in everything! I tried a reboot, but the 20fps issue remained. I am at the end of my troubleshooting skills now. I can't find anything more online regarding these issues so I'm pleading for your help here! I could just re-install vanilla windows (I have a USB installer) and get the drivers from the Dell website. What do you think? Is this the only way to go? Thanks for your help. Jon