Radio frequency interference (RF Interference) produces a false triggering effect on crank trigger and electronic distributor ignitions. Because your AutoMeter electronic tachometer counts the number of times you ignition fires in a crankshaft revolution, additional random, false triggering causes erratic and inaccurate tach movement.
AutoMeter tachs are internally protected with circuits that filter out a majority of the “noise associated with racing conditions. When this noise reaches a high level, however, the tach sees it as legitimate ignition firing and responds accordingly.
Your car does not always demand full capacity of the ignition system. If radio interference is present, it will most likely show up at the top end, in top gear where the most horsepower and high ignition load are present. Memory tachs will give indication of this condition with abnormally high memory readouts as well.