605 immobilizer problem

[grant1078]

My Peugeot 605 has an intermittant problem of not starting after 5-10 minutes running after it has been turned off.If it is left for 1-2 hours it seems to reset it's self and away it goes again.Once the car is warmed up -no problem.When it won't start there is no spark at the plug leads.There is no keypad to enter any code.Any ideas?Cheers Grant.