Hello, my name is Rusty. Since it's limited to one or two keys, that sounds like an easy fix. Here's what is going on, and how to fix it:
There are velocity sensitive pads that are hit when you strike a key. These can wear out over time, causing them to play the key at full volume, regardless of how hard you press the key. They can also quit making sound altogether, and can be intermittent. They are behind the keys when you open the piano up. You'll need to remove the top of the piano to expose these contact strips, and you can replace them at home. They are a maintenance item, and are fairly inexpensive.
Unscrew the 3 screws at the back of the piano. They are the screws near the top of the piano that holds the top lid in place.
Now, we can slide the top lid off. Loosen the lid first. Do this by grabbing just above the lid on one side, then lift up and slid out a little bit towards the keys. Repeat for the other side. Now it should be easier to slide the lid off, and place it somewhere safe.
Remove these plastic covers along the piano cover track on each side. Take the cover off.
Now strike a key, and note where the back of the key hits. It's a little rubber pad connected to the circuit board. That rubber piece is what needs to be replaced, and your velocity sensitivity will return.
You can order the parts directly from Yamaha.
UK: 0844(###) ###-####/p>
If all A notes, or all D notes are out, that would indicate a more serious problem. Since it's three random keys, the pads are 100% the problem, and once you replace them, those keys will play like new.