Replace the delay() with wait(). That function has the same outcome in your sketch but does not block the Duino from executing code in the background (witch is what MySensors is taking care of for you). In your case the acknowledgement from the controller is not processed by your node because you stopped the Duino for 100ms. A wait for about 20 to 50 ms should do the trick easily! And to answer your second question: yes, it is possible that it is a power related problem. Do you have a large-ish capacitor (4,7 - 10µF) neat the power pins for your radio?