I took care of someone with dementia (not Alzheimers - there are many kinds). He would have sudden, shocking losses of cognitive functioning (couldn't remember how to use a phone, near-inability to speak, getting lost, delusions, etc.), and with therapy he would fully recover. Until he didn't. Another family member did have Alzheimers, and she functioned, including driving, during her lucid periods.
I can see Tom veering from capable to non-functional multiple times.