95 90Q fuel mix woes continue - help with any hypotheses besides vaccum leak?

[Joel Lenorovitz]

So I've pretty thoroughly tested my intake system for leaks with my homemade smoke
machine system (pictures to come).  I found no significant leaks, but if I pumped
enough pressure into the system I would eventually induce a leak around the plenum. 
We're talking some pretty decent pressure, though, and something's got to give
eventually, right?  I've already performed "Damien's Fix" (replacing the seal) so
that juncture should be in pretty good shape.  The bottom line is that there does
not seem to be an obvious vaccum leak and my check engine light/driveability
problems still exist.

The symptoms are:
- check engine light coming on
- engine hesitating/missing - anywhere from a minor blip to a significant power loss
- DTC's of lean fuel mix in both banks
- DTC's of new O2 sensors with readings also suggesting a lean fuel mix

The symptoms are sporadic and can't be reliably recreated.  They happen most
predictably when either A) trying to accelerate under heavy load or B)when
decelerating from highway speeds and engine goes from moderately high RPMs to idle. 
Many times engine operation is perfectly fine under these condidtions, though.  I
notice that if I switch the ignition off and then back on after getting the check
engine light (with vehicle still driving), normal engine operation will be
instantaneously restored until the problem decides to recur later on.

I also have a periodic stalling problem at idle which may or may not be related.

Here are my latest hypotheses:
- mass air flow sensor is bad (please, say it ain't so!)
- fuel pressure regulator isn't working properly
- idle stabilization valve is malfunctioning - I've read that this produces a
"controlled vaccum leak", perhaps it's gone out of control and is causing the
problem

Please offer any other ideas and/or comment on the likelihood, means of diagnosing,
and fixes for any of the above.  I feel like I'm one eye-opening discovery away from
getting my car running well again so any help is greatly appreciated.  Thanks,

Joel

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