this problem has existed after converting from manual to a 500 series power box using a GM "type 2" pump.
the assist is fine [way better than the "armstrong" box was] however, going down the road, it seems you always have to correct it.
at a minute correction, it seems the box wants to "keep correcting", then you input a minor correction the opposite way, and the box keeps correcting, rinse and repeat.
the pump is over driven now, [this setup is somewhat of a kit application. it came with the bracketry to fit the pump to the engine, but no pulley] but i can't find out what the original ratio was for the pump. it was a four cylinder pontiac application.
i got a flow valve restrictor to try, so we shall see how that goes.