hi,
wow! you certainly have replaced a lot of items so therefore most causes would have been eliminated. the only other suggestions i can make are if the code is for a map sensor then there is either a problem with the vacum connection to the sensor i.e. blockage in the manifold or hole in the vacum tube etc.. OR there could be a wiring problem between the sensor and the ecu ,this could be a short circuit or open circuit etc, you would need a wiring diagram and a multimeter to check this. OR there could be a fault with the ecu that is causing this code to come up.
the ecu on an 850 is very easy to remove and if you have access to a car with a known good ecu you could swap them and check if that is the cause or not.
hope this helps.
steve.