Hello Huan,
if you are using 32bit counters for octets instead of HC 64bits counters (that can be not available, for example they werent in C6500 with IOS) you can experience a wrapping effect:
when the counter makes its turn around 2^32 -1 it can cause to see sudden falls in graphs that are not real, because the graph is made by substracting from current counter value the older one.
As you can see from the graphs these low peaks are quite regular.
you have 200 Mbps.
it takes:
roughly 160 seconds to
4 10 ^9 bytes / 200 Mbps / 8
to wrap
but your frequency is higher in the order of 10 minutes or more.
However, I would check the MIB used to make the graph.
Hope to help
Giuseppe