We recently had a local with the same problem......changed spark plug wires and it went away.

one of my gaffrigs does the same thing, sent it back and they say all is good. all connections are good and I am not doing much more trouble shooting. I really just think they are crap gauges, mine are 185 hrs old and the needles are faded. I keep my boat in a building. So I just got to where I sync with my ears.