My girlfriend has an automatic '93 Volvo 850 GLT that shudders and
stalls when it's wet; both when it's actually raining and even if
it's just misty.  The car always starts fine, even in heavy rain, but
after driving a couple minutes it stalls.

It usually happens on her morning commute if it's damp out - it
never stalls if the engine was already hot or the car was running
before the rain started (even puddles don't present a problem then).

If she starts it in wet weather the idle is rough: no real peaks, but
it'll dip below 500 before it catches itself.  It doesn't stall
during the idle - only after driving for a couple of minutes.  The
tach will bounce around, dipping and spiking up to 3000 for about half
a minute before it stalls at a stop sign/light.  On two occasions
she's noticed it bouncing and pulled over before it had a chance to
stall.  After about 20 minutes of rough idling the car seemed to right
itself and it ran smoothly again through the rain.

Last time it was wet she tried racing the engine (keeping it between
2-3000 for a minute after ignition until the engine was warm), but it
still died a couple minutes after leaving the driveway.

After it dies it won't start right away, regardless of how long you
crank the engine.  Engine starter does nothing.  It takes about a half
hour of just sitting there before it will start again.  It always
starts again though, even if it's still raining.  After it's
restarted it runs fine in the rain.

I suspected a cracked coil but I couldn't see any sparks when I
looked under the hood last night.  Today was nice and dry, so I started
the car and tried spraying the engine compartment with water while it
idled.  I emptied the entire spray bottle onto the engine but no
sparks.  I even tried spraying the grille to see if water droplets were
being sucked up the air intake, but it kept running smoothly.  The idle
finally got a little bit rough when the spray hit the area around the
belts (passenger-side of the engine).

We took the car out to see if driving would cause it to stall and sure
enough, it died at the first stop sign.  I'm outta ideas, any
suggestions?