Miss in 87 Subaru RX

[Benjamin Marsh]

I have an 1987 subaru RX with the mpfi turbo engine.
The problem occurs when the engine is at normal operating
temperature but not always - driving along at moderate
loads and higher the car sometimes misses, backing off
doesn't fix the problem and the car eventually rolls to
a stop. If I wait until the car cools down sufficiently
it goes again just fine - until it heats up. There is also
a fair amount of fuel smoke associated with the car stopping
which leads me to believe it is an ignition problem.

Over the past 6 months this has happened three times.
1st I replaced the O2 sensor - problem went away.
2nd I replaced the O2 sensor - problem went away, until
that evening when it happened again - I replaced spark plugs
and again problem went away, that was about 6 weeks ago
now it has happened again...

I have a computer fault code of 33 which corresponds to
a speed sensor malfunction, no other fault codes show up
the SS fault shows up all the time, when the car misses though
the ECS light flashes on and off - the speed sensor is still the
only fault code shown. Last time it happened I had a boost gauge
in the car which showed that during the miss I was getting full
boost at full throttle even though the tach was showing at below
1000rpm this indicates I am still getting fuel into the engine
which is going through into the exhaust and spooling the turbo.

Does anybody have any idea what could possibly be causing this?

BTW my local Subaru dealer has no idea - he says he has less
of an idea than I do due to not having seen one of these cars
before I brought it to him...

Thank you for your help

Ben Marsh