A mesoscopic model of a diblock copolymer is used to study the stability of a uniform lamellar phase under a oscillatory shear flow. Approximate viscosity contrast between the microphases is allowed through a linear dependence of the (Newtonian) shear viscosity on monomer composition. We first show that viscosity contrast does not affect the composition of the base lamellar phase in an unbounded geometry and that it only couples weakly to long wavelength perturbations. A perturbative analysis is then presented to address the stability of uniform lamellar structures under long wavelength perturbations by self-consistently solving for the composition and velocity fields of the perturbations. Stability boundaries are obtained as functions of the physical parameters of the polymer, the parameters of the flow, and the initial orientation of the lamellae. We find that all orientations are linearly stable within specific ranges of parameters but that the perpendicular orientation is generally stable within a larger range than the parallel orientation. Secondary instabilities are both of the Eckhaus type (longitudinal) and zigzag type (transverse). The former is not expected to lead to reorientation of the lamellae, whereas in the second case the critical wavenumber is typically found to be along the perpendicular orientation.