Has anyone dealt with memory problems that come & go & what I mean by that is one day she can't remember but the next she can remember all.

She’s not faking it. It’s a degenerative disease attacking the brain. That means that some days synapses are firing, and other days they are not. They will fire less and less as time goes on. There will also be possibly an increase in behavior since their reasoning skills are crumbling. My MIL yesterday was eating cake out of a tall glass and when I asked why and did she want a plate instead, she flew off the handle asking why I’m always bothering her and that it’s her business. Of course, it’s just a cover up because I could clearly see as she stared at it that she had no idea why she put the cake in a glass and not on a plate. People get turned off the behaviors and don’t want to rock the boat, and so let behaviors slide. Fair enough. There’s no changing her besides getting some medication to deal with anxiety.

Patience, and give yourself a pat on the back, because it’s not easy.

Treat her as if there is memory issues all the time, say that's great you are having a good day but we are still going to use those wipe off board for reminding and or alexa...... wearable watch like I have on my dad with alarms.
Eating reminder, bathroom reminding. Everything else technically don't need to remember unless she needs to feed a cat.......

This is very common with folks who have any one of the dementias. And no she is not faking any of it. Her brain is broken and she will continue to have good days and bad until she progresses far enough along that she will no longer have good memory days. So enjoy those "good" days while they last, as unfortunately they won't last forever.
If you are her caregiver, the best thing you can do for yourself now is educate yourself about the disease. Teepa Snow has many great videos you can watch on YouTube about Alzheimer's/dementia, and the book The 36 Hour Day is very helpful as well.
I wish you the very best.

Yes, my dad had vascular dementia and I often wondered if it was a proper diagnosis.

One thing that I would encourage you to do. Tell her when she is being nasty and hateful that it is not okay and if she doesn't stop you will be leaving. Then if she doesn't stop, walk away, give her a time out. Increase the time until you find the sweet spot. When you come back, do not bring up the issue that caused you to leave. If she gets ugly again, you walk away again.

I have seen Alzheimer /dementia patients come to an understanding that they don't get what they want from me unless they aren't mean and ugly. It is like dealing with a bratty child. You tell her it's not okay and you are leaving to let her calm down and when you come back, you expect her to be nice. Do this every single time and even if it doesn't stop her, it keeps you from being her scratching post.

Welcome to the forum!

I think if you read more on this site, you would find that there is no one 'absolute' in the aging process. Except that it gets worse, or maybe I should say 'different' all the time.

My mom, not formally dxed with any kind of dementia, most definitely has issues with memory and keeping things in the timeline they belong in.

Last time I saw her, she was watching 'Bonanza' and commented that Michael Landon was a great friend of her brother's. Which, actually, is/was true. She said "Oh, he was just over to the house to swim last week". Um.....I said "Mom, Uncle D died 3 years ago. Michael Landon died about 30 years ago." This was met with fierce arguments from her, so I pulled up my Google on my phone and showed her. Still didn't believe me.

It made me VERY aware that she is slipping, and doing so faster than ever. We try not to let it bug us, and none of us really believes anything she says anymore-it's pointless and frustrating to all involved.

One of the telling SYMPTOMS of Alz. is just what you described: the brain is no longer smoothly going ffrom point A to B and on. All the messages are scrambled. And depending on the day, the time of day, the many, many things that go into make us who we are--a person can be completely different on Tues say, than they were on Sunday.

It's possible to fake it, but it would take a lot of work. I'd go with the Dr's dx and treat her as such.

Ignore the hateful things. They just come with the territory!