1993 legacy wagon. Brake light and battery light come on at same time and then go away and come back again seemingly randomly. I tested battery with engine off and it read 12.7 volts. With engine on, it read 14 volts, but periodically while idling, engine would make a light click noise and the voltage would quickly go down to 13 and quickly climb back up to 14. And when I turned engine off, voltage went back down to 12.7 but after only half a minute or so went to 18 and stayed there for roughly 10-20 minutes before returning to around 12.6. Any advice appreciated. Thanks.