120 = 12.0 Volts with the engine off means your battery is low and/or bad. This means your battery is only about 25% charged. Either it is low or likely has a bad cell.
You need to get your battery charged and tested, however, if it over 4 years old, time to likely replace it!
Low batteries in these cars makes them go crazy.
Probably time to buy a battery.
DL49 is what many claim fit most everything except convertibles or M3.