We report oscillatory shear and steady-shear measurements on a model main-chain thermotropic polyether in its nematic and isotropic phases, and in the nematic-isotropic biphase. The nematic phase is viscoelastic on all time scales measured. Since these time scales are longer than the molecular time scale of diffusion, we conclude that the nematic viscoelasticity is controlled by the large-scale defect structure. The isotropic phase approaches the response of a viscoelastic liquid, but there are much longer relaxation times present in the isotropic phase than would be expected for a flexible polymer. Viscoelasticity of the nematic and isotropic phases show very weak temperature dependences, but the viscoelastic response of the biphase is extremely temperature sensitive due to the first-order phase transition. The Cox-Merz rule was tested and found to apply in the isotropic phase and fail in the nematic unless the sample is sheared just prior to the dynamic analysis.