It's usually a fault with the brake pressure sensor on the ABS/ESP unit but without knowing what fault codes are stored it's only a guess.

The hunting could be anything from an air leak to a faulty sensor/throttle body etc.

As all the systems are connected via can-bus an engine fault can register a fault in the ABS/ESP.

Need fault codes read in engine ECU and ABS/ESP.

Is the engine management light on too?