We report measurements of the electric field vs current density [E(J)] characteristics in the mixed state of amorphous Nb0.7Ge0.3 microbridges. Close to the transition temperature Tc the Larkin-Ovchinnikov theory of nonlinear flux flow and the related instability describes the data quantitatively up to 70% of the upper critical magnetic field Bc2 and over a wide electric-field range. At lower temperatures the nonlinearities of E(J) can be described by electron heating which reduces Bc2 and leads to a second type of flux flow instability, as shown by a scaling analysis of the high-dissipation data.