Sorry, I meant the 6.4L engine

If you haven't any sort of noise (air in system or noisy vanes in the pump) associated with the loss of power assist at idle then the more probable answer is that your pressure relief valve isn't doing it's job, the valve spring or the bore that the valve and spring sits in is wiped out. Do have someone check the pump output pressure because there might also be a problem with the steering gear box internal piston allowing fluid passing the Teflon o-ring seal. Usually when this is happening, you'll leak a really good amount of fluid out the end cover of the gearbox. If you're going for the inexpensive "throw parts at it" I'd replace the power steering pump. Make sure you check that it comes with that pressure relief valve or you can swap over the old one because in most instances it's the bore of the valve and not the valve itself that wears out causing the valve to stick in it's bore. A1Cardone has a replacement pump for the 6.2L engine at $42.84