Does anyone have a better understanding of Parkinson’s Disease than I do?

Dad can occasionally walk well, as if he doesn’t have Parkinson’s Disease. This has only happened when the physiotherapist or occupational therapist comes over and at the last visit to the neurologist. He won’t leave the house without his wheelchair because he says he can’t manage with his walker. At home, he’s often shaky and unstable, shuffling, as I’d expect with Parkinson’s but I’m confused by his occasional ability to manage better.