a strange one - auto start on stop/start doesnt always start.

[roadwarrior]

My wife reports a situatiuon where her car ( mostly when warm) will stop at traffic lights/traffic ect but when it tries to auto start it always fires but doesnt catch, so tries again, fires but does'nt start, after 3 attempts it will say start it yourself, but of course by that time your getting hooting and angry stares and gestures. It will always start from cold and its intermittant so doesnt always do it,.
Its as if the cpu is getting a signal that the car has started and so cuts the starter motor, any thoughts ? a sensor perhaps ?

there are no lights or errors picked up so im stumped as to where to look.