immobaliser fault?

i have a nissan primastar 1.9 dti van and have a intermitent problem with starting the van i put the key in and the light just flashes and its hit and missif it starts. i've changed the anteni and still dose it . i'e had it in to nissan and they said it started for them so can't do anything .thats the problem itfires up no problem but when it dose'nt i can be there for nealy 1 hour be for it starts. fustrating? any ideas......