About a year ago one of my two FP2s started presenting touchscreen erratic behavior (screen freezing and erratic keyboard, dragging and scrolling functions). Initially I thought it was due to the screen overheating, but the real cause was somewhere else: static electricity combined with a defective screen.
Screen freezes and erratic touchscreen behavior happened when the phone was connected to some wall chargers or on buses and trains, and the dysfunction seemed to be getting worse lately. In order to troubleshoot it, I used one charger that reliably caused screen freezes with my defective FP2, but that caused no problems to my other working FP2. I switched the screens and found out that the defect moved to the other phone, meaning that the problem was located on the screen module. Cleaned and adjusted screen contacts and electric pads, but it made no difference.
Then I realized that the reason for wrongly thinking that heat was the cause of the problem was that I had a garden lounge chair made out of plastic that produces body static electricity. Since the problem frequently occurred when I was sunbathing, I thought it was related to heat, but the real cause was static electricity produced by the chair. At that point I also realized that some clothes would also cause screen misbehavior, and the reason was that they were made with materials that produced body static electricity.
This could be confirmed by observing screen misbehavior when using the previously working phone with the defective screen while resting on the mentioned outside lounge chair.
Ordered then a new screen from Fairphone, and as expected this entirely solved the problem: no more screen freezes with the problematic charger, and no more erratic touchscreen behavior due to electric interaction with cloth or seat materials.
Happy to give this old FP2 that still looks like a new phone a few more years of life!