Again, I am thinking that it CANNOT be a dirty contact on a key or keys.
My rationale is that it is EVERY C# and G note. If there was gunk under one contact, I am unsure how that would make all other notes of the same pitch sound at max volume.
This keyboard senses velocity by having two contacts under the key at different heights. The pressing of the key depresses one, then the other, and the processor determines velocity by the timing between the first and second contact hit.
When the person above said the C# and G are on the same row, what did that mean?