Unit has intermittent engine/exhaust brake malfunction. this is causing the vehicle engine/ exhaust brake to disengage when on a decline at highway speeds. as result the vehicle picks up a great deal of speed and could cause the over use and eventual failure of the vehicle's air brake system. as well as the inability to control the vehicle (though application of the air brakes and/or steering) on a downgrade. there is no warning prior to engine/exhaust brake failure. this originally happened to me while traveling on east bound i-80 over donner pass summit heading towards truckee, ca. and has happened 3 more times since then. the local volvo truck dealership can find nothing wrong with the vehicle.