[LeftCoastErik]

I have a perplexing issue (at least to me) that has recently cropped up. Here's what's going on...
I had Palo Alto Speedo make me a tach. Shortly thereafter (and since verified unrelated) I noticed that sometimes the tach would be bouncy and almost at zero at idle. Sometimes it was perfect, but it was inconsistent.

For reference, my car has very little electical load. All LED lights, fuel pump and ignition are all the constant-use components.

I recently added an electric heater to the car. Direct battery power with a relay to switch it on.It has a variable fan, but pulls 40amps max.

If I have the car running, and I turn on the heater, the tach will go from flaky to dead nuts perfect. While watching the Megasquirt gauges, I also note that the ignition advance is flaky at idle, just like the tach, but perfect with the additional load of the heater.

In addition, and this is new, when I shut the key off, power to the gauges fades away over a few seconds, and the car keeps running for a few seconds, like a relay is slowing de-energizing. If the heater is on at the same time, it shuts right off like it should.

I have a USB socket with a digital voltmeter built into it on the dash. It charges at 13.8/13.9 at idle. Headlights drop it 13.7ish. Adding the heater will pull it down to 12.9/13.1, but it seems like its doing its job.

I have checked an cleaned all grounds.

Is this a sign that the alternator is going away? Voltage is maintaining, but maybe the amperage output is weak?
Thanks for reading!