Your .pdf data looks like it is very badly aliased

Good point, these data are at or near the Nyquist. I'll set the JPI
recorder to 2s intervals and collect some data this way to see how
different it looks.

Do you see ammeter or voltmeter fluctuations? Might there be a halogen
beacon operating at the meter frequency?

No halogen beacon, just an old-fashioned incandescent blinker at about
a 3s duty cycle. No strobes, either. The JPI is my only voltmeter.
Ammeter bounces around a bit in concert with the beacon. Other current
draws are the usual suspects, 2 Garmin radios, Garmin transponder and
audio panel. These were both ay flights so only the beacon was on all
the time and taxi/landing lights on for takeoff and landing.