Tachometer and speedometer are bouncing

What could cause the speedometer & Rpm needles to vibrate then drop to zero and the emergency brake light to come on then go back to normal? While driving the car the speedometer and RPM's will fluctuate rapidly then drop to zero, the emergency brake light comes on then goes off and both needles return to normal. The other night was extremely cold and it happened three times while my wife drove home from work.

She said the lights dash and headlights seem to flicker slightly but she isn't sure if the headlights dim or other indicator needles drop when it happens. The battery, starter and alternator have been tested and are good. Happens on the highway or on the town streets, there are apparently no patterns. Drove around town myself yesterday; hit bumps, traffic lights stop and go traffic and there were no problems.