I had this with my Ford KA. Mine was a 1996 model. The fault on mine was with the Engine Management System. A Ford garage plugged in a computer and adjusted it.
I've actually just got rid of my KA. The fault had started to happen again having not seriously misbehaved for a few years.
My advice would be to take it to Ford, let them fix it. Be very assertive and ensure that you're happy that they have found and fixed the problem (not just reset the computer as this could mean the fault will come back in a few months time.)
Without wishing to depress you, this is the sort of problem which seem to be tricky to fix. Start noting down who you speak to and who says they have fixed what, because you may find that you'll need several attempts to fix it and you might not want to continually pay for Ford to experiment on your car. (If you can show that you have paid to have this fault fixed before, and it has not been fixed, you may not need to pay again.)
Luke
September 2004