Sounds like the usual failure of the anti backflow valve inside the water pump. There is a posting a little further down that is the same issue.
http://www.thorforums.com/forums/f8/...psi-15496.html
You hear the pump short cycling because the anti backflow valve is leaking.....so pump pressurizes the pipes and shuts off....but water leaks back to the fresh water tank through the leaky valve in the pump which causes a water pressure drop that starts the pump again. Suggested solution is to turn off the city water and run the water pump a few times to clear any obstructions in the valve. Then try turning on city water again and see if your overflow continues. The other option is replace the valve in the pump; replace the pump or you can purchase a stand alone anti-backflow valve and plumb it in series with the pump (make sure you orient the valve in the correct direction if you choose this option....but its a cheap solution with a bit of plumbing required) .
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