I measure about 12.5 V and it drops a bit when I turn on lights. Nothing out of the ordinary. I suspected the blower as well so I checked with the engine running but the blower off and I still saw the voltage jumping around.

I'm thinking it is likely the ignition since increasing the engine RPM seemed to make the voltage spikes go away. It is possible that my meter just doesn't measure the spikes when the frequency goes up. I replaced everything but the coil in the ignition about 5 years ago. I haven't had a chance to check the ignition yet.