Too much info missing from your end:
-gpu
-room ambient
-chassis ambient(always warmer than room ambient, but to varying degrees)
-chassis fan setup
-Cinebench version and length(R15, 20, and 23 are not the same, and each newer one being more up to date than the last)
-motherboard settings: things like manual/adaptive Vcore, and Load Line Calibration setting
What can be relayed from the current info provided thus far:
1)Cinebench isn't a cooler thermal test, but a Voltage stability one. The load fluctuates too much to test cooler stability.
-Prime 95, Small FFT, AVX/AVX2/AVX-512 Off, 15-20 mins. Try to stay below 80C, but don't exceed 85C.
-OCCT, Small Data Set, Normal Mode, Steady Load Type, SSE Instruction Set, 15-20 mins. Try to stay below 80C, but don't exceed 85C.
Those would be the go-to thermal stability tests.
2)The current chassis may hinder your overclocking endeavors.
All coolers' efficiency is affected by how much air the chassis allows through it, and for the S340 Elite, both intake and exhaust are strained; can't get larger volumes of fresh air in needed for the OC'ed part, and can't get the extra heat produced from OC'ing out as effectively.
TL;DR: Even if the missing info were corrected, it does not look like you have much room to work with.
I feel like I forgot something...