Motivated by recent experiments on the electro-hydrodynamic instability of spin-cast
polymer films, we study the undulation instability of a thin viscoelastic polymer film
under in-plane stress and in the presence of either a close by contactor or an electric
field, both inducing a normal stress on the film surface. We find that the in-plane stress
affects both the typical timescale of the instability and the unstable wavelengths. The
film stability is also sensitive to the boundary conditions used at the film-substrate
interface. We have considered two conditions, either rigidly attaching the film to the
substrate or allowing for slip.