My OEM volts gauge normally sits right in the middle, but I get these spikes up to about 16 volts. Spikes last up to 30 seconds or so, then it drops back down. No real pattern to it, pretty random. Lights get really bright. Also, lights flicker a bit at idle.
I checked and cleaned up the alternator ground wire.
I have a nippondenso alternator, previous owner said it was recently replaced (last few years?).

Is this a voltage regulator issue, can it be replaced? Any checks I can do?

 

yes the VR is on the fire wall about 3" square 3/4 " thick. not real expensive

 

Try loosening the two mounting screws on the VR, then retightening. Could just be a poor ground. Even cheaper than the cheap VR.