If you are getting these extra notes without any kind of harmony feature or automatic accompaniment going, then this does sound like something going out of whack on the keyboard. Additionally, the notes you are describing do not sound like anything that would automatically be played by the keyboard, even with auto accompaniment, as they make up a "suspended +4th" chord (G, Db {or technically, it would be called C# in this case}, and D is a G sus+4 chord, and Db, G, and Ab is a Db sus+4 chord), and I seriously doubt that those chords are supported by the auto accompaniment in that keyboard, and even if they were and that was the cause of the problem, then you should hear it on notes you play in any key, not just those two combinations.
Therefore, you may want to try a factory reset of the keyboard. Of course, you'll lose any data you've saved, though I don't believe this keyboard has much in the way of registrations or user song recordings, so that may not be a concern. In any case, you may want to give it a little while longer to see if anyone else here has any solution that would not involve a complete factory reset, and then if no other solutions work, you may want to try the reset. If the extra notes are still sounding after that, then it may very well be a defective keyboard.