Advice on periodic voltage fluctuation - C172

Recently I have noted an in-flight system voltage fluctuation in my
C-172K. The fluctuations are quasi-periodic, with an amplitude of about
0.4 volts and a period that varies between about 1.6 and 1.8 minutes.
The varations are contained within a band between 14.0/13.9 and
13.6/13.5 volts (the monitoring system quantizes to +/- 0.1 volts).

I have posted a graph of these observations for two 1.5 hour flights at
http://home.comcast.net/~argon39/Vol...luctuation.pdf

The battery is less than a year old (conventional Pb-acid Concorde),
the voltage regulator is a quite new Zeftronics unit. I replaced the
regulator about a year ago when I first noticed fluctuations (the
replaced regulator was a 1969 vintage electromechanical unit that was
original equipment).

I know of no quasi-periodic electrical load that has a similar duty
cycle, and thus the observations would seem to point to something in
the alternator (brushes and diodes in the alternator have not been
worked on in quite some time).

Any ideas about what might produce these quasi-periodic changes? Thanks.