Oldman said it best.
Tank pressure will stay at ~960psig until you run out of liquid co2. At this point, the upstream pressure(the pressure of the tank) is flucuating so quickly(as it decreases) that a single stage regulator can't reliably control the discharge of gas.
As mentioned, a high quality needle valve can control this, as well as a pH controller, or a two stage regulator.
Personally, I use the pH controller.
Alternatively, you can monitor cylinder pressure, and when you see it go below 700 psig, take it in for a refill.
