2002 Citroen C5 keeps dying on the road

I have owned cars since 1980, and never experienced ANYthing like this. Since last December, ie for 6 months, our 2002 Citroen C5 Exclusive has repeatedly died on the road. The engine gets jumpy, the message comes on saying Anti-Pollution fault or Automatic Transmission fault, etc. Then the car goes in to a shop (several different shops). One time, the problem ''went away'' only to return a week or so later (a shop in So Jutland kept it a week, drove it around, found no fault); four times, the shop (the licensed Citroen dealer here in Aarhus) has found some expensive problem to fix - drivetrain replaced, air intake housing, hydraulic line, ignition ''spole'', etc. - only to have problem return again. I have spent thousands of dollars on this, with no satisfaction. It does back to the dealer again - this is the SIXTH time - with no end in sight. We have not paid for visit #5, and will not until the darned thing is REALLY fixed. The shop themselves seem frustrated. WHAT could be happening? What should I do?