OK, so what SD cards are recommended? I guess read/write speed is crucial, right?

I don't think poor ventilation is a problem. It occurs no matter how long I use the device. What do you mean by bad SD card?

@paradadf you are right.

Pi3 running at 1200, no overclocking, latest Recalbox.
I have tied two different emulators, different cores.
No lighting symbol. What would that mean?
It's not TV related - sound stutters the most.
It has happened ever since in every N64 game. Whenever there is 3D rendering it goes nuts.

Should I install it all again?

Guys, what should I do? Playing N64 games is a nightmare. Sound statters, games lag, everything's terribly slow...
PI3+