Help please ! Problem 323i

1997 BMW 323i suddenly stalled at 70 KM/hour (40m.p.h.) for no obvious reason. Started again after a few minutes. Brakes were functioning throughout. Electricals seemed to be otherwise functioning. Dealers ran diagnostics and could find nothing wrong. Can any one offer suggestions as to causes or how to diagnose a possible fault please ? This has happened about 4 times in 6 months. I've Lost confidence in the car. Thanks.