My car keeps cutting out?

could any1 help me please, i have a fiesta zetec 1.4 2003 reg, ive just changed my alternator belt and my secondary belt, my tensioner and a pipe on the engine which had a split in it, then the garage phoned me and said it is still cutting out and losing more air, and he has plugged it up to the computer and its finding no electrical problems, anyone know why?
thanks