I have a Y reg (2001) Ford Focus Ghia.

It is a 1.8L Petrol.

When you turn the car on and let it idle - or if you stop in traffic, the revs seem a little low. It's almost like it's struggling to stay turned on.

Sometimes the battery light switches on, as if it's stalled, although immediately goes out, the car kinda "coughs" and runs the same.

It has once or twice entirely stalled. But re-starts with no issue.

Do you know why this is happening?

idle control valve is the likely suspect there im afraid though dash cluster faults are also common as is a collapsing vacum hose to check the hose pop the bonnet with the engine running and find the hose with the bend in it that goes into the rear of the throttle body now give the car a few revs and watch the hose if it gets sucked together its collapsing and will be causing youre issues also at the front where the exhaust manifold is theres a hose connecting to a metal cylinder this is the pcv valve simply pull up and it pops out the spring weakens in this causing issues but its likely the icv valve found on the pass side of the engine bolted near the top and side with 3 bolts holding it on easy to replace youreself