90 100q stalls out

[syljay]

Problem solved!!
3 out of the 4 downpipe/catalytic converter bolts were missing.

Diagnostics:
I attached a voltmeter to the O2 sensor connector and observed the
voltages during startup and warmup.

The O2 sensor voltage started of at 0.599 V.
In the next 3 minutes it steadily dropped to 0.077 V.
In the next 3 minutes it rose slowly to 0.33 V.
No voltage swings.
When engine was revved the O2 sensor voltage would swing in the normal 
fashion.

I let the engine warm up for 13 minutes.
The O2 sensor was now swinging between 0.2 V and 0.9 V . . but the 
swing cycle was only once per 7 seconds.

I shut off the engine, jacked up car, put wheel stands under wheels, 
went under the car for a looksee.
Three of the four downpipe/cat bolts were missing.

Replaced the bolts.

Restarted engine.

O2 sensor voltage starts off at 0.59 V, and in less than a minute the 
voltage starts swinging in the normal fashion --  0.2V <--> 0.9V every 
2 seconds.

Took car for a spin and duplicated the time/distance when the car 
stalled out. Engine worked fine.

Conclusion:
The loose downpipe/cat connection allowed air to get into the exhaust.
The O2 sensor either did not get hot enough to work properly, or was 
giving the computer wrong readings.
I suspect the O2 sensor readings were erratic enough for the computer 
to disregard the O2 readings and put the engine into "limp home" mode.

I visually checked the spark plugs. Plugs looked normal - no
carbon or sooting  . . . which suggested normal or lean conditions.

I suspect that when I restarted the car after the 30 minute store visit:
1. the temp sensor told the computer the car was warmed up, therefore 
keeping the air fuel mixture normal and added no enrichment.
2. The computer disregarded the O2 sensor readings and went into "limp 
home" - - - which is a bit on the lean side.
3. The lean mix might have caused the stumbling . .and the no start 
condition after stalling out.

Thanks to the guys who offered advice. The ones who suggested air/fuel 
mixture issues were on the right track.

SJ in NJ