I'am having a problem with my 2005 fiesta st.
I have fitted a full 4-2-1 milltek exhaust, cosworth cams, k&n.
The car worked perfecly for about 70000km but now on 87000km,
the idling of the engine is working properly but when I let revs drop from 3000rpm, revs drop too low to 500rpm and sometimes engine cuts out.
Sometimes it takes couple of tries to get it running again.

Does anyone have any ideas why is this happening?

I have checked the car does not have any
DTC.