Oscillatory unstable behaviour in a SAG mill is characterised by the mill constantly cycling between an overload condition and recovery from that condition by a substantial reduction in feed rate tonnes. Such behaviour is commonplace with SAG mills and gives rise to a significant overall reduction in average throughput which might be as high as 10%. This paper provides a theoretical analysis of the causes of oscillatory behaviour in SAG mills using conventional control system theory and modelling. The paper is illustrated by trend plots taken from a SAG mill in a major gold mine. The paper goes on to explain the practical causes of the instability ranging from inappropriate type 2 control strategies to poor control loop tuning and to inappropriate operator intervention caused by lack of operator training. Finally some practical solutions are given in the form of guidelines for the tuning of conventional controllers or expert system controllers, and in the form of guidelines for operational staff to achieve a steady state for a SAG mill.