Our beloved van "Cinco" (an 85 purchased for $5k on Cinco-de-Mayo)
stalled on the freeway yesterday returning from an otherwise
wonderful vacation.  We managed to tow it home with the help of a u-
haul auto-transporter, a cable-pull, and a lot of grunting, swearing,
& sweat.

I'd greatly appreciate help with a diagnosis.

The details:
   85 Westy 1.9l automatic transmission.
   150k miles on engine.
   One head replaced by PO (not sure of mileage)
   Has long-term minor head leak (exhaust smells a bit like coolant,
burns about 1 liter / 200 miles)
   Has low oil pressure at idle, which is tolerable with 20/50 mobil 1.
   Within last 5000 miles has had:
     new (rebuilt) AFM
     new cap & rotor
     new plugs
     new radiator
     new fuel filters (has both kinds, pre- (NA) and post-
(California) fuel pump)
   Has a rough idle, and tends to stall unless you give the idle
adjustment screw a few turns open, in which
     case it tends to idle at 1500rpm.
   Runs fine on the freeway -- gets about 18mpg @ 60mph
   Passed california emissions just barely (HC was near the cutoff).

The event:
We had just driven over the grapevine and were cruising along I-5.
It was evening, and temperatures were dropping.   Car was driving
fine.  Stopped for gasoline.  Got back on the freeway, and was
driving fine.   10 miles later, power abruptly cut w/o warning.
Pulled to side of freeway.   It would crank just fine, but not start
(not even a cough).  Checked all fuses, hoses, pulleys, etc.   Had
car towed to a gas station.   After unloading from the towtruck,
tried to start it, and it immediately started, then immediately
stopped within 2 seconds.    The engine sounded completely normal
when it started, and it cut off abruptly (it didn't sputter to a
halt).  It wouldn't start again.

Diagnostic info:
   Coolant was about an inch below top in the main tank, but there
were no signs of overheating.
   All spark plug wires and look good, resistance is within specs.
   Rotor & cap look quite burned for being fairly new.
     Note : these are cheap third-party cap & rotor, however.
   Fuel pump runs and appears to have good pressure (as judged by
gasoline shooting 5 feet in the air
     when cranking with the pressure-T screw out.  Yes, that is a
stupid way to test it!)
   Did the Bentley 35-pin connector test.   All results were
unremarkable, i.e.
     injector resistances, throttle closed & open switches, ground
Ohms, etc.
     Note: At one point, the "left relay" test did not appear to have
12v when ignition switched on.
     I repeated the test later and it did, so I'm not sure if this
was my error or a vital clue.
   Appears to have spark when cranking (removed sparkplug and
grounded it while cranking).
   Sparkplugs are wet & oily looking, but this was only noticed after
cranking it for a long time
     w/o starting.  Normally the plugs are light brown.
   During multiple starting attempts, had two backfires.   These were
apparently explosions
    in the crankcase (?) as smoke popped out of both valve covers,
and in fact the right valve cover
    was blown off it's seat.   Put the spring back on and it sealed
back up ok.
   The left back set of grounds was somewhat corroded.   Cleaned and
re-tightened them but no change.
   Through perhaps 50 starting attempts, perhaps once or twice it
would cough for a 1/4 second but
     never seemed to even give the hint of starting up.

Theories:
   We initially suspected bad gasoline, since the problem happened
about 1/2 gallon away from the gas station
     under otherwise mild conditions (air temperature dropping,
cruising at steady throttle).  However,
     the pump seemed to have good pressure (though we've not actually
measured it with a gague).
   Other folks have suggested "bad grounds" / EFI as the problem.
The fact that the car did start after
     being towed and sounded totally normal for 2 seconds and then
wouldn't start again does sound electrical
     in nature.
   Not sure what to make of the backfires / crankcase explosions.
Possibly just a side-effect of repeated
     cranking w/o starting?  Or could this be gas in the oil, caused
by injectors running too rich?

Ideas / Suggestions?

-Mike