So as usual, my 'bird has developed multiple problems at once. Sometimes it runs fine but about half the time when I drive it now I get ignition noise in my stock radio (a high-pitched buzzing noise that increases w/RPMS), along with a jumpy tach (worse at idle), and my dash lights/radio display/headlights will flicker a bit as though the voltage is inconsistent. At speed the voltage is ~13V, but at idle it jumps around 11-14 volts, which I haven't noticed before. As for the tach, I'm not hearing or feeling a change in actual engine RPMS, and my laptop that I have hooked up to my EBL ECU doesn't show spikes when the tach jumps around. These problems either come all at once, or don't show up at all.
Also I doubt it's related, but it seems like I have to put a little more effort in my leg now in order to keep the car from moving when I brake; could just be my bad knee acting up again though. And I think I have a vacuum leak somewhere as my ECU sometimes says my IAC is fully closed and yet it still idles a bit high (800-600ish).
The battery is a few months old, and I did a tune-up (ignition module, wires, rotor, cap, fuel filter) 3 years ago. The plugs were replaced about 1.5 years ago. The air filter is an aftermarket open-element that I put on 3 ago, and the starter was replaced 2 months ago.
In an old post here, someone said that when his tach was jumpy, it turned out that his alternator was bad. But before I go dumping time and money into figuring out the issue, I was wondering if anybody else has had this same exact problem? I'm guessing I'm hearing ignition noise in my radio because the tach isn't happy, and the tach, voltmeter, and lights aren't happy because the alternator's dying & putting out inconsistent voltages; or there's a bad ground somewhere.

 

might want to check the nut that holds the wires onto the starter. Had a similar problem today and it turned out to be loose wires

 

And engine grounds too.

 

I seconded checking your grounds.