Something is wrong with Fords EGR setup on the Zetec motors. The EGR opens up too quickly under partial throttle (i.e. normal driving) and causes severe stumbling/studdering/hesitation. The cure is a small vacuum reducer which makes the EGR open up slower under partial throttle. This was the best money I have spent on my car so far.

Another cure would be to remove it all together which I plan on doing eventually.