Good afternoon, I hope I can help you.
There is no point in trying to guess what the fault is. There are too many to choose from that will have similar symptoms.
You need to get a garage to connect a suitable scanner and see what fault code is stored and also check the live data to make sure all the sensors are working properly before you can accurately diagnose the fault.
You are doing the right things but it's surely cheaper to get the fault memory checked and get the code than it is to keep guessing and replacing parts that are not at fault.
My experience is that it's the o2 sensor, but without proper testing it could cost another £50 to replace it without a guarantee it will cure fault.
You don't need a garage to repair the fault, just get the code. Should only take them 10 minutes at most and then you can decide how to proceed.