As peterleinchen says. That it happens whilst actively using the phone is definitely not a coincidence. The battery's internal resistance increases as it ages. The voltage on the battery's terminals equals the internal voltage minus the voltage drop. And the voltage drop equals the current drain times the internal resistance. So it should come as no surprise that the extra load while actively using the device should cause enough viltage drop to cause problems. I once had a phone that lasted 7 days on standby but would reboot when I received a call with the battery 90% full. A new battery was the solution. Get a new one.