My Dad was babbling total psychotic nonsense, screaming it at the top of his lungs, for a long time to me on the phone. Then I heard that he was talking to the security guard a little while later, completely normal. He needs to be in a mental hospital again. (He was in once in 2009, because the police were called when he threatened to stab a neighbor).
He needs to be put on some anti-psychotic drug, like Risperdal! But he refuses anything except valium, which he gets from a cardiologist, as he has refused a Primary Care doctor for decades. He also has congestive heart failure and is afraid that they are going to find out how much alcohol he drinks.
His own brother is a retired family doctor, and he can't talk my dad into going to see a doctor.
If they can really control the craziness this much, to focus it on their children and some strangers, then who is to say they really aren't insane, that they are just acting because they don't think they will have any consequences? They lost the ability to care, and so they are just letting it all hang out?
It is SO frustrating when they showtime to relatives and then the relatives say, "I just don't SEE IT in him." They don't believe me and that's maddening, when he's a babbling psychotic to me.
I agree that the elderly have been given a ticket to abuse under the diagnosis of dementia. They lose all empathy for others, so they just fire away at them, their kids and caregivers who sacrifice the most. They lose the ability to care what other people think, so they abuse strangers on the phone. But when they have to be good for the doctor, they all sweetness and nice. And that robs the doctor of the ability to make a correct diagnosis and give treatment.
I am staying away from the alcoholic father and am going to Al-Anon. They all are supportive there too, and understand the frustration with dealing with the FAKENESS.
So in a sense people with cognitive problems may appear to be able to "turn it on and off." That doesn't mean they don't really have serious cognitive problems.
I don't know if this applies to your father or not!
I suspect there is little that can be done for him until the next episode that involves the police.
This must be very painful and frustrating for you. I am so sorry.