My Mom can become totally confused and can't express what she is feeling, needs or wants. She simply cannot find the words. Then 30 minutes later, she is telling me who our local NFL team is playing this week or who won the local MLB game last night. I don't understand how this comes and goes. Is there something that triggers it?
UTIs in the elderly can cause all kinds of symptoms that clear once the infection has cleared.
Another possibility is that she's having TIAs, sometimes called "mini-strokes".
When was the last time she had a comprehensive exam by her PCP with you present to report symptoms?
If the fogginess tends to happen more so in the evening, then that is called "Sundowning". Or does the fogginess happens just before eating or after eating? Have a notepad handy to write down what you notice. Then look for a pattern.
I know for myself and my sig-other [we are both 75] that we tend to be tongue-tided at breakfast time. It's like our brain is still half-asleep.