Citroen Saxo 1.1 Forte 2003 Immobilisor light flashes?

This has happened on 3/4 occasions when the engine stalls and the immobilsor light flashes.After about 5 minutes it will start and the car moves ok.
had it checked by my garage and no fault shows up on the diagnostic computer.
It is very frustrating as the car will stall anytime in the middle of a journey