System voltage could be changing by 0.00001 volts and that could make the display change from 13.8 to 13.9 as that is as much resolution the display has. Likewise with rpm, low resolution, changes in 10 rpm increments.
Oil pressure is different, it has 1psi resolution (I think), so might have a flaky sensor.
What happens if you select a cruising rpm say 100rpm higher or lower?
Got any DC loads running that have variable draw, like autopilot? That can cause volts to vary.
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