Ford Galaxy intermitent power loss

when running the engine in my ford galaxy 2.0 s reg sometimes its hesitats when you press the accelerater or if the car is pulling of from traffic lights there there is no power for time from a few seconds to few mins and the engine will sometimes backfire but after a while the engine picks back up and runs normaly this usually only happens when the engine is cold and only in 1st or 2nd gear and never doesit while in 3rd or 4th under normal driving conditions