Tim,
I have only seen this once before.
There is a flow measuring device in the inlet of the water heater. The heater only fires when there is flow enough. That flow device is a plunger with a spring and a switch on the plunger. When the plunger gets sticky, there is no flow. That poor little pump is trying to do its job and finally the plunger moves, but now the flow is higher than the continues capability from the pump. So after the flow device opens and relieves the pressure, there is now not enough flow to the inlet device shuts up again.
If it was that the pump had just lost some capacity, then the water is supposed to flow but at too low a rate for the heater to fire.
Good luck
Matt
