1500W is a little more than what most 2' columns can realistically handle. Use of a diffuser plate to help level out the surging should help because it will allow you to reduce the setting on your hotplate, but the plate will average the heat. The other option is to bypass the thermostat and build a regulator that will allow you to effectively reduce the wattage without any cycling. This is not expensive ,most 1500W reostats or a voltage regulators will do this for you,, and not really difficult. The great advantage to the 1500W, though, is that it will heat you up to a working temp very quickly.

It is best to keep the heat input as constant as possible, as any change here will have to work its way through the entire system, requiring time for the column to re-equalize. Try to keep your cooling water cold, and again, at a relatively constant temperature and flow rate. If the water does warm slightly, you can counteract this to a certain degree by increasing the flow rate.