There's nothing much you can do about it, it is pre-programmed to send out a pulse every 15 seconds or so, and it depends on where during this 15 second sequence you turn on the flow, if you turn your flow on when it has just sent a pulse, you will have to wait 14 seconds for the next one.
I think the pulse senses/ measures the amps the the pump is drawing, obviously if you have the flow turned off, the pump tries to draw more amps to overcome the pressure, when the drawn amps reach a certain level, a switch is triggered in the Varistream and turns off the pump, it will continue to send out a pulse every 15 seconds or so, once the amp demand drops, the switch is triggered again to turn it on.
Funny thing it never does this when you first turn the Varistream on, and your system's not pressurised.