I have a much simpler Casio keyboard- CDP-130, but have experienced a very similar issue. Intermittently, the A1, F1, C#2 and A4 make no sound, and send no signal to midi. As we were pulling the key action section out of the case, the dead keys suddenly sprang back to life. Slightly twisting the entire key bed/board would duplicate the failure. So without risking the snapping of plastic tabs by completely taking it apart, I vote for a bad solder joint or micro-fracture in the circuit board holding those keys. We bought the CDP second-hand, so it's possible it was damaged at some point or perhaps even stepped on.
