Like blissz mentioned, there is some amount of normal fluctuation in difficulty that will cause low points and spikes in the hashrate chart. Your chart, however, looks more extreme to me. It looks like your miners might be constantly restarting. The first 3/4 of the chart is mostly normal, but the last part (where the hashrate is bouncing between 0 and a high number) looks like something is wrong.
I'd check the status page on your firmware to see how long they've been running. If the time is low, then they're probably rebooting.