A study of finite amplitude instabilities occurring in cellular premixed flames is undertaken, both in the case of 2D and 3D flames. We use two different models: the complete equations of isobaric flames with one global exothermic reaction and a modified Michelson-Sivashinsky equation (including gravity effects). Different instabilities are found that can be explained in terms of Eckhaus type instabilities of cellular patterns. An oscillatory instability also exists, which produces self turbulizing flames