What version of NAudio are you using? The ASIO support in NAudio is working much better than it used to, but it wouldn't surprise me if there are still some interfaces it has problems with. ASIO has many different ways of presenting the samples to the interface, so it might be that your interface uses one that hasn't been well tested yet.
Do you need to be using AsioOut, or would WaveOut or DirectSoundOut work for you instead?