Repairs to digital piano keyboards?

Definitely a contact problem. There are two contacts that are triggered in sequence when you hit a key, one as the key begins to drop and the other at the end of the key motion. The computer uses the time between these two events to calculate how fast the key is moving, and thus, how loud the note should play. If the first contact is dirty or not working correctly (such that it is late or absent), the notes will trigger louder than they should. As for the DIY cleaning, I did it once on a keyboard I didn't care much about but I can't advise on whether you should attempt it on something as nice as a kurzweil.