I've never experienced this kind of problem, but I would start by testing
the Hall sender and Hall control unit per Bentley. That's where the signal
for your tach originates.

[Slightly off topic, but has anyone but me stressed over the tiny size of
those wires from the Hall sender to the connector inside the distributor--on
which the entirety of engine operation depends? They are also absolutely
unfixable...]

Long shot, but if your distributor Hall connector is bad (the plastic
sometimes breaks) the sender wires can ground out or short, wreaking havoc
with the ECU. Don't know if this would explain the erratic tach behavior or
not but it's worth a look. With the engine at idle, you might try moving
those wires around with a pair of needle nosed pliers to see if there's any
effect.