With cyclic data, is is nice to get the true rate of change over time. As Miner states, if you are using an automated adjustment, you have already implemented institutionalized over-adjustment. The adjustment becomes the process, and masks the true process variation. Is it possible to turn off the auto-adjustment feature? At least for a period of time you could actually determine the actual process variation. If you have a true cyclic variation, Shewhart charts are not adequate to provide control. They expect independent, random variation, and you will have a function variation.