I'm finding my iMac 21" late 2015 to be unstable after upgrading to High Sierra If I leave my iMac for a while, it refuses to come out of sleep mode; the screen remains black, the mouse, once it unfreezes moves slowly and then the pinwheel appears forever. I'm forced to force power down (pressing the button). 

To explain :

I've just bought a reconditioned "as new" 21" iMac which came with High Sierra, but unfortunately the setup was all in German despite my choosing the UK language.

Apple support advised me to erase and reload my OS X, but of course my last registered OSX is El Capitan. All went well with that and the migration of my files from Time Capsule backups, but I thought I should follow Apple support's advice and upgrade to High Sierra.

It hasn't really worked well since. It's sometimes very slow to respond despite it's fusion drive, while at others it works fine.

Any ideas? I think I'd rather return to El Capitan which seemed to be okay...
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.