Is yours a 1.6? I think you may want to look at the recall lists, some focii were recalled due to ECU faults that caused the engine to lose power. Below is the recall date and description from an article I purchased from Used Car Expert
"20/03/2000 1.6 Engine may lose power due to a malfunction in the power train control module software."
Ford should update this for free if you return it to them, obviously a 2003 model should have been fixed but...
Will Marsh
July 2006