The electron transport through a short period modulated two-dimensional electron system exhibits clear negative differential conductance (NDC). The modulation parameters place the NDC peak clearly into the validity range of miniband conduction and it is therefore attributed to Bloch oscillations of electrons in the lowest miniband. In contrast to conventional superlattices the NDC in our device is stable for low carrier concentrations due to the reduced dimensionality of our system. With increasing electron concentration instabilities occur in the I–V-traces at the onset of NDC. Numerical simulations confirm this transition and a stability criterion depending on the system parameters is given. Further evidence for stable Bloch oscillations is given by coupling the device to an external high frequency field. For fixed frequency a clear suppression and shift of the peak current are observed with increasing intensity of the radiation. Both facts are predicted by the semiclassical theory when Bloch oscillations are frequency modulated by an external AC electric field.