Fire tablet keeps shutting down during use. It's fully charged.

My table is 100% charged yet it sporadically shuts down while I'm watching a show. At first i thought it was the apps so I force stopped them and then my charge went down to 1%. The next day when I opened my tablet it was up to 79% charged even though I didn't charge it overnight! Usually I have to hold the power button to off for 40 seconds then let it rest for 5 seconds then turn it on again. I've been doing this for the past year but am getting tired of it. Any suggestions?