Volvo Repair: 1991 740 turbo stalling, power stage, bell housing

Question
my volvo stalled last week I replaced coil because there was no spark, car
started right up drove fine for 2 days and it is now staliing on ocassion
and starts right up what is the possible problem and how can I repair  

Answer
Tony, I would replace the Bosch power stage on the firewall.  Its a square part with a plug on it.  That controls the spark.  I would also check the condition of the engine speed sensor that sits on top of the bell housing on the transmission.  If its frayed replace it.  Also clean and adjust the throttle body.  Roger