Timing belt was changed out about 10k miles ago.  That would be...
scary... to say the least, if it were that.

Plugs changed also.  Also, both front side oxygen sensors (not the
catalytic monitor, after the catalytic converter).

What has not been done is valve lash, which is daunting, from what I
can see of the procedure.  Basically have to strip the entire throttle
assembly and plenum to do that.  I am not sure... would bad valve lash
present a symptom similar to this?

That thought occurred (and I suppose a valve lash will have to be done
sooner or later), but it would not present as an intermittent condition
if it was valve lash, I wouldn't think...?

IAC valve is a real possibility.  That would not code if it's sticking,
only in case of electrical failure.  Friend of mine's 4 Runner had
that, but the idle in his case was erratic and prone to dropping off
during starting.

I tried the knocking things loose technique, using a length of hose
and... just... sucking on it like a straw.  EGR holds vacuum (diaphragm
is OK).  It did not sound as though it was moving at first.  After a
while, you could finally hear the pintle whack into the seat when
vacuum was released, little metallic click.  I haven't had a chance to
road test it since then.

My thought was it was stuck just a little bit open, and exhaust gas was
getting through.  However, for this theory to be true, a few other
things also have to be true...

First, the car can't be able to throw a P0401 / P0402 code... Too
little / too much EGR gas.  It can't have a monitor for this condition,
or I would think it would have coded already.

Second, whatever the EGR is dumping in can't be enough to cause the
fuel trim to go out of range.  It must be small enough that the
injector could compensate, or you'd get a code once again.

So first of all, in my thought process here, and for future reference
as well, does the car have the ability to throw a P0401 / P0402 code?

Second of all... how do you clean up an IAC valve... and how can you
tell if it's sticking?  I've never looked at one, just seen the effects
of a failed one on my friend's 4 Runner.