This looks like trouble in your code. You gosub to "check_button:" and then goto "button_loop:" if a button is pressed. This creates a case where the gosub never returns, which can cause problems. Beer may help.

Code:
check_button:

        if btn_up = 0 OR btn_dwn = 0 OR btn_back = 0 _  ' If any button is pressed 
            OR btn_nxt = 0 OR btn_entr = 0 then         ' Go to button loop

            High lcd_lite                               ' Turn on the LCD Backlight
            goto button_loop
        endif
        return