Didja ever look for that allen screw ? I think the model # has a 4820 in it.
I bought it brand new 8 months ago, and it did exactly as you described. I brought it back to the dealer, and he called the factory, and they came back w/ the screw adjustment......I believe it was 1 turn CW. All I really remember is, it corrected it perfectly. These pumps that have this vari flo adjustment so that when using very little water, they don't hammer like the old pumps did. It is supposed to take the place of installing an accumulator tank.But they still don't work all that well when a faucet is bearely opem.