Should I replace my voltage regulator?
 

Drove up to the lake on Friday, noticed the headlights and dash lights would randomly get brighter then back to normal, completely unrelated to rpm. Ok, voltage regulator, I thought. Drove it over the weekend including some track time while carefully monitoring voltage (checking while idling with a multimeter) and battery temp. All good. Drove it home yesterday, headlights didn't change brightness once. Should I still replace it, or was it maybe just a one-off thing? Similar weather/temp, if that makes a difference.

I'd presume your vr/alternator is failing and replace -- when mine failed, the voltage spikes were pretty random and since I didn't know better, ended up frying my tach and DME before I fixed. Now run a voltmeter all the time (via my cigarette lighter) so I can detect my next failure

Probably the VR is going bad. You can get a meter to plug into the cigarette lighter like darrin mentioned for pretty cheap if you want to verify. That's what I did when I had the same symptoms you're describing and sure enough voltage would be normal/steady and then jump to 15+ volts for a few seconds. You might consider getting alternator rebuilt once you have it out of there as well.

Voltage Regulator
 

It’s cheap insurance. http://forums.pelicanparts.com/uploa...1571186808.jpg

Ok, thanks guys. I'll pick up a voltmeter and keep an eye on it.

I'd recommend that you first check your battery and the area around/below your battery for moisture/evidence of overcharging -- these voltage spikes canl cause your battery acid to "boil" and damage the surrounding area. If you see any evidence of this, I'd recommend not driving the car until you have your alternator/vr rebuilt/refurbished.

Another "tell" of a failing voltage regulator can be phantom illumination of seat belt warning light -- again caused by voltage spikes. My concern here is that your vr may have been letting your charging voltage spike for quite some time (especially if you're not always driving with the lights on) and I experienced first-hand how continuing to drive with these voltage spikes can damage a variety of electronic components.

a lot of parts stores will test the charging system for free.