If a stator diode blows, it usually blows closed, resulting in backfeed of current from the battery, through the primary cable to the starter, to the B+ terminal, then the wrong way through the diode into the stator, then then the RIGHT way through the trio diodes into the D+ wire, then back to the warning lamp and into the ignition switch, then into one of the accessories also connected to ignition terminal 15, then through the accessory, then to ground! Lighting the warning lamp when the key is OUT! Whew!
Of course there are three stator diodes. A failure of one OPEN will make that phase of what's supposed to be a flat DC waveform drop out, resulting in a scallop. You could see this with a scope. Here's a good link to a page with some bad diode waveforms illustrated.
http://www.omitec.com/en/support/waveforms/
In any case, at very low RPM, this would probably cause a flutter in the needle. It's something else I expect.
Leaving the ignition on with the points closed can burn out your CDI.
Why is the meter wandering? Have you removed all the oxidation from the spot where you are taking the measurement?
My guess is you have a bad ground somewhere that's intermittently coming in and out as the car vibrates.