Depollution faulty on full time?

I have a peugeot 207 vti petrol. Have depollution fault and check engine light in my car for over one year now. I have changed the spark plugs, both pre and post oxygen sensors, fuel pump plus its two sensors; tried fuel additives, changed air filter, pollen filter/air element/ a/c cleaner and nothing has changed.

I even got rid of the catalytic converter but this helped only for few days and the fault is back. The car sometime has a very hard start, rough idling in the morning, loss of power, cutting out on low speed, losing power on high speed and failing to pick up when in traffic. Kindly help with what to change next now that the car is still with the dealer and nothing seems to be working. Even the peugeot dealer is now running out options on what to replace now that they say the software is just fine.

ken
May 2017