How can i stop my ford ka's engine from cutting out at low revs?

Hi,

I have a ford ka 98 reg, not every time but a fair amount of times when im braking my engine cuts out.

I have heard a lot about the idle control valve so iv tried cleaning it with no result. Then I replaced it with one from a scrap yard yesterday and engine just died with it was fitted. So today i got another one from scrap yard and car works fine but the problem is still there.

The guy at the scrap yard went off to test the latest one he gave me on an engine so it should be working. So is there a way of checking my supply to the valve or the valve itself, or is the problem nothing to do with the ICV in the first place.