[Mark78]

I have a ford focus 1.5 petrol st line. Sometimes when im starting the car the revs are dropping to around 500 and car is cutting out.

[NMNeil]

Any local garage should be able to help you, all we can do is guess.

[JamieGen]

I had a similar issue a while back and it turned out to be a loose battery terminal/earth lead - something so simple but it caused the whole system to reset. Definitely worth a 5-minute check under the bonnet to make sure everything is tight and there’s no corrosion on the leads before you start spending money on parts!