citroen berling V reg no starting?

Citroen Berlingo v reg. Diesel, when you put the key in the ignition the inmobiliser keeps on coming on and off and bleeping, I had citroen to reprogram the key code but they check the key and inmobiliser, it hsows that the key is reading the code and that is not the fault, what else could it be. sometimes it starts but as you try to pull off it starts bleeping and then the van dies out. citroen dont know what is wrong until they put it in the garage and give it a thorough check which will poss. cost more than the van is worth.