Display warm-up: It’s crucial that you let your display warm up before attempting to calibrate and profile it. Although CRTs were always known to require a warm-up period to stabilise, it is also true for LCD panels, though once warmed up they then remain more stable than CRTs. Allow at least half an hour.
Energy and screen savers: Disable anything that is likely to change or shut off your screen before beginning profiling. Most screen-saver timings are based on user input activity and won’t necessarily notice that a calibration application is running.