I'm finding my iMac 21" late 2015 to be unstable after upgrading to High Sierra
It's highly unlikely you'll have an instability issue on one OS and not the other. 

I'd focus on getting High Sierra installed and running since Apple will support that the best for you. Also - you could wipe and reinstall with no apps, no data - just use the brand new install for a day or three to be sure the system is stable. 

If not - you'll know to seek a return / price adjustment from the seller or at least know you need to fix the Mac and it's not a software issue if you do a clean install and it's not working.