The exact same thing happened to me with a spotty connection at the diode board. It's the connection at the right (looking at it) where the large wire plugs into the hot side.
If the battery shows 12.5 volts, do a load test. See how far the battery drops when hitting the start button - shouldn't go below something like ten volts.
Wanted: Dead, smashed, crashed or trashed gauges
BMW GAUGE REPAIRS - TACH*SPEEDO*CLOCK*VOLT METER *PODs & LIGHT BOARD*