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Fiesta Mk6 loss of power, juddering, hesitant idling

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Probably it has been previously discussed here, but I can't seem to find what I'm looking for so I kindly ask for help! To me, my Fiesta is dealing with some serious problems. A year ago it suddenly started hesitating when on idle, there was a severe loss of power and slumpy acceleration. Since then I've been trying to find the solution. Various DTCs have appeared on the scan tool: CAT low efficiency, cylinder 4 misfire, TPS circuit A low voltage, IAT sensor high input, etc. Mechanics and repair shops couldn't help me at all. Parts that were changed during the last year:

- Spark plugs

- Fuel, oil, Air filters

- Ignition coil ( No more P0304 - Cylinder 4 misfire and the power of the car is back again. The old coil was badly cracked on multiple locations.)

- Fitted a second-hand throttle body & MAP sensor from a scrapyard ( P0222, P0123, P0113 appeared after the first time I changed the TB and MAP, but later disappeared. Considering the car runs better with them both fit. )


After the coil change, I cleared all of the DTC and the car was a rocket! I thought the thread was solved, but actually, it turned to be partial solving. Suddenly, out of nowhere the same problem resurrects. Check engine light on, only P0420 this time. For example, on the streets when idling it judders several times, then the RMPs are back in normal range----> green light ---> slumpy acceleration. This occurs with the spare TB & MAP installed. I put back on the original ones and the result was the same. The o2 sensors vary together in the same range, but the post-cat varies in large margins when the load is increasing.
IMO it could be down to 3 or 4 causes: Catalytic system ( CAT and/or o2 sensors) needs replacement, Intake leak, Faulty wiring harness and/or damaged ECU. Also, there's a strange hissing coming from underneath the airbox, but I can't seem to tell if it is an air leak or the injection system's noise. I don't want to use any propane, smoke machines, etc. on my own because of safety reasons.
If someone has encountered the same problem, could please tell me what to look for? I learned a lot trying to fix the problem, but I think I've reached a dead end with this reoccurring fault. Thanks! 

Fiesta 1.4 16V, 2002, petrol 

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