I'm more of a fiber channel guy but can you verify there aren't any QoS configurations applied anywhere in your iSCSI infrastructure? It almost seems as though it's being throttled down or "policed" after a certain period of time. What other performance monitoring solution(s) do you have? As mcowger said: take a look at ESXtop.
What else is running on this SAN? Is it possible something else is chewing up I/O or bandwidth randomly?