while it might not be the same issue as yours, I remember running into this problem years ago with my i7 2600k.
You already ensured the thermal was applied correctly (that was issue 1 for me), second issue was the heatsink I had was inadequate, combined with a hot room in general and my computer exhaust too close to the wall it made things kinda bad.
So I replaced the standard heatsink with a hyper 212 evo and I saw improvement (my computer was not shutting off from overheating). Next I checked my case fans to make sure they were facing the right direction, turns out I mounted my intake fans wrong so I fixed that. went from a constant 75-80c idle down to a 45-50c idle.
That's what worked for me back then, perhaps you are in the same situation.