Intemittent trouble starting

[MissC]

I have an 08 plate zetec Fiesta.  Recently I have had an intermittent issue with starting the car. Most days its starts fine, and on the odd occasion it starts and the revs drop like a stone and it cuts out. If I rev the car a few times it stays running. 

This only happens first thing in the morning,  and has happened when there has been a full tank or even very low amount of petrol.  

The garage couldn't find anything wrong ... said the fuel and air filters were fine, nothing on diagnostics. 

 

Any help or advice would be appreciated.  

Tia