[T3] Problem starting / warming up

[Joshua MacDonald]

Dear type3ers,

My '70 fastback has recently begun behaving as follows (in Mountain View, CA):

It turns over and fires up, revs to idle speed, then abruptly stops firing,
the engine speed free-falls by maybe 25%,
then it starts firing again. The process repeats for several
minutes--idling up, free falling down, ...

The engine sounds normal on the upswing. The free-fall sounds just
like it does after cutting-off the ignition.
If I depress the accelerator, the engine revs faster but free falls
just the same.
After a couple of minutes, depressing the accelerator will rev to a
higher RPM than at initial start-up, but still the same free-fall.

Eventually, it resolves, I assume because the engine has warmed up.
During the idle-up/free-fall cycle, I possibly hear the sound of a
relay switching on/off under the dash. It's also possible the the
battery voltage is a bit low.

Any ideas what the cause may be?
Thanks,
Josh