The biggest issue with the stock MFI breather setup is that it is very easily affected by vacuum leaks. A crap oil filler cap for example will introduce a leak as will the pipes going hard regarding the breather setup or even the auxilliary air valve pipe. It has been said that even a leaky crank seal can cause the running to go on the piss. Naturally these vacuum leaks will have a knock on effect with the running of the car.

My advice? If the car is relatively stock ensure there are no leaks or iffy hoses, breather filters etc. and only use genuine Ford bits ; I have never known on a pattern oil filler cap to last more than a few months before the rubber starts falling to bits.

I would also check your warm up regulator to ensure it is working as it should. Whacking it in the fridge will soon have you knowing if the disc is OK in addition to whacking 12V on the terminals ; it has a disc inside that upon opening introduces an air leak to aid the running of the car.