Fairphone 3 turningoff and restarting randomly but constantly

Hi to both and welcome to logo,

I would think this is likely a problem with internal contacts, including those of the battery itself so I would advise you to start with those, they are easy to access without disassembling the whole phone. Turn off the phone, remove rear cover and battery. Clean the battery contacts AND those of the battery emplacement with a cotton bud soaked in surgical grade alcohol (90° available from chemists/pharmacies) or isopropanol. Leave to dry for an hour before replacing the battery.

If that doesn’t help, then remove all the modules so that you can clean all their contacts. Use the iFixit Teardown as a guide, but there’s probably no need to go beyond step 7.

Beyond that, we’re talking problems with solder micro-fissures, components etc. For that, I would advise getting in touch with a Fairphone Angel if there’s one nearby, who might be able to help track down your problem.