Immobiliser operating when engine hot? (MGB 1974)?

I have not had the system switched on for some time (as I have a battery isolator), however I suspect that the immobiliser could be cutting in (through a short?) after the car has been running for a while. Once it cools down, the engine restarts. I have replaced the most common parts that could cause this problem. Has anyone experienced a similar problem please?

Geoff Trigger
April 2010