Volkswagen Polo car wont catch, sometimes revs itself

My polo is a W reg 1.0L. Every now and again it refuses to start, it's as if it wont catch.. but it always eventually does. Sometimes it catches and 'revs itself' up and down randomly before eventually cutting out. It does it at random times.. sometimes when it first starts and sometimes when it's stalled at traffic lights, therefore not a temperature problem.
It recently had a new alternator, and also a new coil and plugs. It also went to a garage and because it appears to be an intertmitent problem they could find no fault. [b]Please help![/b]