Puma Immobiliser

I have a 1.7 Puma, which intermittently refuses to recognise the ignition keys (the immobiliser light blinks very fast on putting either
of the two keys in the ignition). After a period of time the fault clears and the car behaves as normal (with the same keys). I have had it tested in a Ford dealer using their diagnostic equipment, and they find nothing wrong. The say they can't immobilise the immobiliser!
Anyone else had this problem?
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