Nitro 5 Overheating and a white flash goes across the screen any suggestions?

You first have to open the back cover of the laptop and the cpu/gpu and their fans and paste needs checking if everything is in good order and functioning 100%, also, clean the laptops vents. Overheating in very rare cases can also be cause by internal power cpu/gpu voltage circuitry issues that are shorted and not functioning properly and need to be diagnosed by Acer techs or an experienced tech in your area, because if you leave the laptop as it is, the laptop can sustain critical damage and be unusable as the motherboard will have to be changed. Check all that out and if the first cleaning and repasting doesn't work then have your Nitro 5 fixed and check by an experienced tech, as all those symptoms should not be happening, btw I've got a Nitro 5 model that is the same age as yours and its never ever overheated in everyday usage and/or playing demanding-cpu/gpu games on this laptop but, I've upgraded the ram to with 16GB dual channel and have also changed ***** OEM PCIe 3 x4 M.2 boot drive to a faster WD SN750 PCIe 3 x4 type M.2 drive and all works perfectly.