03 plate 206 2.0 deisel engine immobiliser fault on dash?

my 2 litre deisel has intermittent fault and not starting or turns over a few times before starting.
engine immobiliser fault comes on dash computer, but
it has been fine for 2-3 weeks after not starting at all.
it goes gradually worse over a couple of weeks before not starting at all - ive took the contacts off i can see and cleaned, but no help :-(