1994 1.1L Fiesta stalling problem

I am having an intermittent problem with a 1994 1.1L Ford Fiesta (HCS CFi) that I recently bought.

Even when driving, the engine can sometimes stall. It sounds like the engine is not getting the correct air/fuel mix, as if the automatic choke isn't working properly.

I have replaced the air temperature sensor that ultimately controls the auto choke to no avail. On suggestion I have also had the single point fuel injection unit replaced, but still the problem seems to exist as intermittently it still sounds like it will stall. On an older car I would think that the tick over rate or choke needs adjusting but that isn't possible as this is controlled electronically.

It has also been suggested that although a long shot, the fuel cap may be dodgy and causing a pressure build up in the fuel tank, therefore affecting the flow of fuel into the engine.

If you have any experience of such a problem or any advice it would be very much appreciated.

Thanks
Steve