To me, that sounds like frontotemporal dementia -- specifically Logopenic PPA. The long pauses are a dead giveaway (if it wasn't just acting on her part). The swings in day to day functioning are also really common for frontotemporal dementias. Eventually PPA looks like behavioral variant FTD as it gets worse, and bvFTD is a fucking nightmare to deal with. Your frontal lobes (which hold impulse control, ability to reason, access to short term memory, and executive function), turn into mush and all of the patient's worst impulses come out, but their long term memory is intact. Unfortunately in the early/mid stages they can often muster enough cognitive reserve to look relatively normal for short periods and fool doctors/bystanders who aren't familiar with FTD. People, including your average geriatric neurologist, don't know shit about non-alz dementias and even the weirder alz variants (there's frontotemporal variant alz too, along with dementia with lewy bodies and vascular dementia, and some lucky ducks have more than one dementia at a time). You've basically got to get to a teaching hospital or the mayo clinic to get answers.
It's the most common dementia in under 60s (it's what Bruce Willis almost certainly has, Robin Williams had DLB) -- and I've seen some wild stories from caretakers. Hypersexuality, sudden addictions to gambling or alcohol popping up out of nowhere, spending the entire family's life savings on scammers... some of these younger guys end up in jail in locked forensic units long after the sentence for whatever dumb crime they did is over, because there is no safe community placement for a physically healthy 200 pound 45 year old man who keeps trying to rape or attack the staff until he's basically a vegetable.