iPhone keeps crashing and restarting
My iPhone 6s has been crashing and restarting at random. Sometimes it will be while using an app, sometimes it will crash like it's out of battery. It crashed at 30% battery, came back at 10% battery, shut off because it "wasn't charged", and then, once I plugged it in, it said I had 52% battery. I have forcefully shut it down and reset the settings already. I've also tried to recalibrate the battery, as had been previously suggested, but it shuts down immediately from being at 10%, and then as soon as I plug it in, it jumps to 40% or some other high number. I haven't so far been able to run it down to just 1% battery.
I don't really want to get a new phone unless absolutely necessary. Any ideas on how to fix this?
iPhone 6s, iOS 14