From your description the most likely issue is the physical button on the button board is intermittently failing. It is entirely possible that something is causing the button to stick. In either case you will need to remove the button board to check or to replace it. To remove the board follow the steps 1 through 11 of the [guide|41074|Nintendo 3DS XL 2015 A,B,X,Y, Start or Select Button Replacement|new_window=true] guide.