iMac (2015 late 27'') gets hot in sleep and fan turns on and off

[kk123]

I think what happens is when iMac goes to sleep, fan stops, but even in sleep mode (Power Nap & wake for network access disabled) the heat is too much to dissipate passively, so it gets too hot and triggers the fan to run full blast. And every time it wakes up, Macs fan control shows high temperature.

Does this happen to your iMac too?

I just opened it up and did the upgrade, plus cleanup & reinstall, but the issue persists.
Would it help to reapply thermal paste, or is there a way to put it in deep sleep (so less / no heat)?

 

[kk123]

And most of the heat probably comes from GPU when connected to an external monitor (via a miniDP to HDMI cable), Radeon R9 M395 reading goes from 85°C down to 65°C once disconnected.