2005 Vauxhall Combo starting problems

my combo van has starting problems, sometimes the van will start straight away and then sometimes it will start after a few turns,and will then cut out when u slow down or at a junction, once its going its fine, have recently changed the injectors, air flow meter and timing sensor, its as thought the fuel isnt getting through or as if its too cold to start, but once its been running its fine, any help much appreciated???