308GT4 Gauges - unstable readings

While at a steady cruise, the gauges sweep back and forth slowly, about 1 or 2 mm. It takes a second or two from the high point to the low point of the movement. I've looked in the manuals and cannot find a voltage stabilizer.
Anyone ever noticed this or have any ideas? Thanks!

 

Thank you. I did look back through most of the threads, and found a lot of helpful stuff, but nothing with this specific symptom. The speedo also goes to 80 mph and stays there with the ignition on. Hopefully it's related as well.