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.

if you have fitted cams then you seriously need to get it custom mapped.

The car had no problems for 30 000km with cams.

yes but the ECU has now learnt not to like them, no way the engine will be running properly, fueling will be all over the place.

but that's why the car has lambda-sensors so the ECU can calculate the correct air to fuel ratio.

As steve said above or check the throttle body is clean and not gummed up.

I checked the throttle body and its clean on the outside but inside its a bit oily and black

Seriously get a remap it would make the world of difference, cars can still run too rich or lean even if the ecu is meant to learn what the correct air to fuel ratio is

I cleaned the throttle body and now engine works much better. The revs no longer drop under 700rpm but now there is a new problem. Revs seem to "hang" now. For example the engine idles at 1500rpm for about 5sec and then drops to the normal 800rom.
Could this be if the butterfly valve is bit stuck?

I have that since I fitted my 60mm throttle body it revs high to 1,800 then gently sits perfectly on 1,000 revs.

Not sure that it's too much to worry about

Mine also does that after cleaning the TB Hope I aint damaged it

Or could it be that the engine has to get used to the extra air that its getting after the clean up

Engine revs rising and falling is normal. It's to meet emissions.

It will do it slightly more after it's been cleaned as it has to get used to the new air mix (as mentioned).
It should settle after a few 100 miles