[MMXT]

For a longer time I wanted to ask you, if you use a case and/or some kind of active ventilation for your testing machine, and how long you play h265 1080p material?
I use the official Raspberry Pi Case (pink and white) w/o the top enclosure (because I need the pins for Hyperion) and I have fitting little copper heatsinks on each relevant chip and for real material like films I cannot use h265 playback capacities of RPi3, because after ~25 to 45 minutes the overheat-warning permanently shows up in the top right corner and RPi3 starts to thottle down.
This can NOT be reproduced by playing short hevc sequences like big bug bunny and tears of steel. So I wonder if my hardware is somewhat faulty or if it is not just me.

I use a Flirc case (there is also a kodi branded version) that is very effective at removing heat.

You might want to try removing the case you have and aligning the PCB vertically to help with air flow.
Obviously a fan would be helpful (although not needed with a case like the flirc).

Okay thank you very much for telling me.
Just to ensure myself: So you yourself are able to playback HEVC movies ("x265 1080p") in full length w/o running into overheat troubles, right?