Does dementia come and go at first?

Often, yes. It's not clear cut, just as stages of dementia aren't clear cut. Some days are better than others. If you notice symptoms from time to time, then think about sleep patterns, medications, stress and other things that could cause these symptoms. If the person is diagnosed with dementia, the situations mentioned above could make it worse.
With dementia, it isn't like a person is just fine one day and the next day they have dementia. It develops slowly and therefore may at first show signs only at certain times. Be sure to talk to a doctor about this.
Carol

Yes. Mom had Alzheimers and exhibite progressive dementia, but even towards the end of her life she would unexpectedly say things like 'Your shoes are pretty" or "I'm OK, I guess" which left us constantly scratching our heads. The brain is a very complex thing for sure.

Sacline, my Mom's dementia symptoms do come and go. Sometimes she is confused minute to minute, hour to hour or day to day. Then she also has very lucid concepts, maybe for a few seconds...and looses track of where she was going with her thought. Her confusion and hallucinations are becoming more and more problematic for her and for us, too. I guess the best piece of advice I've read is to remember your Mom as you knew her. Love her and realize she can't help her diminished capacity. It's the disease which is the problem you are facing with her. MM said it well, the brain is a very complex thing! You are in good company here. (((hugs))) Bee