Fuel economy, idle, emissions problem -- please help (long)

[Josh Kinsley]

Something ain't right with my car, a '93 classic, stock,
with 120k miles. I've been dealing with the following
symptoms for at least 24 months now:

1. Poor fuel economy. I get about 20 mpg city, 25 highway.
In the past I would typically get about 26 city, 31 highway.

2. Emissions trouble: high NOX. At my last inspection, I
failed with 4.25 GPM (limit 2.50). I was able to pass by
disabling my EGRC-BPT valve and setting timing to 13
degrees, but just barely (2.38 GPM NOX, and my HC and CO got
significantly worse, though still passed)

3. Cold idle/stalling problem. The engine has a tendency to
stall. It seems to be more of a problem the colder the air
is outside. On a warm summer day, the problem is absent. The
sequence goes like this: Start the car. Idles nicely around
1300 RPM. Reverse out of my parking spot. Clutch in for
first gear. RPMs nosedive, often stalling. Interestingly,
this problem disappears after about 30 seconds of sustained
driving. If I keep my foot on the gas, the RPMs never
nosedive and I don't stall.

Here is a recent history of pertinent maintenance:
A) New fuel filter
B) No codes present in ECM.
C) New O2 sensor
D) O2 sensor switches normally (more than 5 times per 10
seconds per the FSM)
E) Timing set to 15 degrees
F) Idle set to spec per the FSM (750 w/ TPS disconnected)
G) Always fed 87 octane
H) New plugs NGK PFR5G-11
I) New EGRC-BPT valve (filter crumbled out of the old one
during testing)
J) Cleaned EGR tube, per SE-R.net
K) EGR valve bench tests perfectly.
L) EGR solenoid bench tests perfectly.
M) EGRC-BPT valve bench tests perfectly.
N) All vacuum lines replaced with new and in good condition.
O) EGR valve twitches, but doesn't stay up during normal
operation
P) Cleaned throttle body off the car, per SE-R.net
Q) Suspect catalytic converter may be shot due to many miles
of poor NOX emissions

Thanks to everyone who reads through all of this. I'll be
most grateful for any advice.

Best,
Josh Kinsley