iPhone 6 plus when power on after 2 to 4 minutes it power off and on

iPhone 6 plus when power on after 2 to 4 minutes it power off and on itself then repeat the same process again after 2 to 4 min, I changed the battery but it did not solve the problem, I flashed it with updated IOS but still the same and during flashing process it did not turn off…… ..when connected to charger the Vbat voltage fractalated between 3.5 to 4 does not stay constant…what might be the problem????