My phone had similar symptoms for a few months. It was warning up even while browsing Chrome. Two days ago I changed a few settings and so far it seems to have come back to norm. I don't know which particular settings change has helped, but the idea was to make the android as pure as possible. I did these things:
- turn off Asus OptiFlex (settings/advanced)
- switched system preferences to stock android (settings/system preferences)
- disabled Tips and MyAsus (settings/apps & notifications)
- turned off steady charging (settings/battery/battery care)
After writing this answer I checked the back of the phone and it is cool.
