These vehicles will react in this manner if the wiring loom in the gearbox area is damaged (broken/green corrosion) causing a slight resistance in the circuit intermittently. Also the leads going to the battery have to be making a perfect connection on the battery terminals, if not the immobiliser problem will occur. The local dealer has never sold a [pick up coil (around the ignition switch)for these vehicles. On a different theme the low pressure fuel pump is notorious for failing in an intermittent manner on these as well causing similar symptoms to the ones you have. This pump is not in the tank by the way. This pump can eventually and slowly disintegrate while at the same time slowly recking the high pressure pump. Cheers