This is typical of NH. You are not "mining". You are renting your hash power. The zero is when the miner switches from user to user I believe.
while this is true, but somehow wrong, if everything is workig great then you shouldn't notice any disconnect on your NH status page, the current hashrate has to always to be somewhat close to the average, the only thing i can think of right now is that the firmware he uses has some dev fees, that's one common way of the sudden 0 hash rate which will not show on NH if the miner is 24/7 directed to NH.