My tablet is in constant reboot.

My Samsung Galaxy 10.1 is about 3 years old, I have kept up the updates for it, but resently it has been in constant reboot. It turns on, it turns off all the time. And the battery could be fully charged and then when it reboots, it may only have 40% battery left, then turn off and on again, and not it has 85% battery. HELP!!