Battery power cuts out on my Yamaha E423 keyboard

Even with brand new high end Panasonic alkaline AA batteries, my keyboard will start buzzing and the display will start flickering 45 minutes to 1 hour after being turned on. After the buzzing and flickering it will just turn off. Plugged into AC the unit works fine. Does anyone have this problem? Would appreciate your advice? (The Yamaha e423 is several years old. But gets very light use.)