Random reboots or shutdowns are mostly caused by 2 issues:
1. CPU/GPU overheats and to prevent any damage, system shuts down.
2. PSU fails to deliver enough power to the GPU or fails to keep smooth enough voltage for PC's operation.
85°C for GPU is too hot since you'll get thermal throttle at 83°C and at 92°C, GPU will shut itself (and most likely the whole system) down to prevent damage due to the overheating.
So, clean your case internals from dust, speed up your case fans or even add more case fans for better airflow.
When you get your temps within reason and you still experience random shut downs during gaming then it's safe to assume it's the PSU who is acting up.
While your PSU has enough wattage for your system, my guess is that your PSU voltage regulation has sifted outside of the ATX PSU standard specs of 5% on all rails (10% on -12V and -5V rails) and your PSU can't sustain stable enough voltage for your 120W GPU to run under higher load. That's why your PC shuts down during gaming.
Only fix is to get a new, better build quality PSU. Here i suggest going with any Seasonic unit in 500W range. E.g: M12II-520 EVO, G-550, Focus 550 or Focus+ 550,
pcpp: https://pcpartpicker.com/products/compare/TgW9TW,DPCwrH,bkp323,KmgzK8/
Focus and Focus+ are the newest PSU lines from Seasonic and they come with 10 years of OEM warranty. G and M12II EVO series PSUs come with 5 years of OEM warranty.
All my 3 PCs: Skylake, Haswell and AMD are also powered by Seasonic. Full specs with pics in my sig.