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More electrical oddities

Here's one I'd like to bounce off the collective you:

In attempting to more precisely characterize the behaviour of my
'83 UrQ, I have the following non-obvious behaviour. I'd like to
know if anyone else out there can either reproduce it (particu-
larly in an '83/vintage, or any other models as well) or even
rationalize it for me.

With engine fully warmed/etc.: switch on heavy static electrical load
(e.g., fresh air fan to high, A/C *off* -- the compressor and air
adjust completely change the behavior, rear window defroster on);
bring engine to 2500 rpm and hold steady; switch on highbeams;
switch off highbeams. On my car, when the highbeams switch on, the
engine rpm goes *up* 100-200rpm (and the voltmeter drops maybe a half
volt or so), then eventually settles back down to about 2500 again
(5-10 seconds) with voltmeter remaining constant. When I switch off
the highbeams, the engine *drops* 200-500 rpm as the voltmeter jumps
back up, after 4-5 seconds the engine regains most of its lost revs.
It will do this very consistently, like 9 times out of ten. (It is
of course hard to hold precisely 2500 rpm with minimal engine load,
as any body movement -- like reaching/pressing switches -- tends to
cause enough foot movement to change the engine speed noticeably,
but even controlling for this (pushing dash/instrument pod rather
than light switch, for example), the effect is quite consistent...)