I think I would be demanding a new steering rack, motor and whatever control module looks after the steering unit. In my opinion, driving a car with an intermittent fault such as this is DANGEROUS. If the dealer won't play ball, try escalating the matter to Holden Service. I seem to remember another poster had the same problem, and Holden told him the car was dangerous and shouldn't be driven until new components were installed.
My 2015 Caprice started doing the same thing, and I hear of reports of lots of taxi operators having the same problem. It has been blamed on the choice of connector used by the motor manufacturer, and involves some mysterious 'fretting' of the connector contacts in the plug connection harness-to-motor. Detach the plug, clean and inspect, refill with special water resist dielectric conductive (Whatever?!) Gel and supposedly Bob's your Uncle.
Much communications with Holden Customer Care imploring me to take it to the dealer who very annoyingly insist on booking the car in for a full day in the 'shop to carry out a 45 minute procedure, Valuable time off the road for a working car.
Regardless, the main problem is that 'the Computer' insists on turning off the steering Power Assist when the comms link to the P/S motor is even momentarily interrupted due to this mysterious 'Fretting'.
I HAVE EVEN HAD IT HAPPEN WHEN THE CAR IS SITTING IDLING WAITING FOR A CLIENT, EVEN WITH NO-ONE IN THE CAR! It can happen while turning (roundabouts, curves) or on a dead-straight highway at 100+ kmh
I have pushed HCC to find out for me, WHY the system needs to have the ability to turn off the power assist motor. if there is some kind of parts-preservation logic, I cannot figure what. In a crash situation, no-one will care if P/S is working or not.
IT WOULD BE PREFERABLE TO HAVE THE 'SERVICE POWER STEERING' MESSAGE DISPLAYED WITHOUT TURNING OFF THE ACTUAL ASSIST!!!
Nor will Holden let the tech-heads talk directly with a customer to discuss the issue.
Although I am a bigger man, and a professional driver, and able to 'catch' the car when the disable happens, other drivers may not, and could end up in a dangerous or life-threatening situation.
Interestingly, I had noted the problem more likely to occur in hot/dry conditions. Here in Alice we went into an unusual 'rainy season' from Christmas eve until almost Australia Day. I had made arrangements to have the car looked at on 28/12 if possible, but with the inclement weather, the car settled so I plugged on. Australia Day dawned beautiful and sunny, and within a day or 2 the old gremlin was back.