Unstable production behavior affects many offshore fields, particularly at their decline stages. "Casing heading" and "density wave" are two types of instability that have been identified and both can result in production loss. These instabilities have influence on each other, but the interaction has not been fully addressed yet. One contribution of this paper is to study the interplay of these two phenomena. For this purpose, key variable of density wave phenomenon; pressure at injection point in tubing that is governed by an integral equation; is considered as a fourth state variable added to 3-D dynamical model of casing heading. Another contribution is some modifications on the casing heading and density wave dynamic models. Finally, the proposed model is simulated in MATLAB® and OLGA®, and the results are compared with well operation facts and data from field.