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Flux flow instabilities at high current densities have been recently reported to occur in high temperature superconductors. Such flux flow instabilities were already predicted by Larkin and Ovchinnikov. However, other mechanisms may possibly also account for this behaviour, as for instance hot spots or thermal instabilities. To investigate the mechanism at the origin of these instabilities, we have carried out detailed I- V experiments, using short current pulses, on YBCO thin films grown on sapphire substrates. When the current density exceeds 2-3 times the conventional critical current density, we observe an extremely fast transition from a low dissipative state into a highly dissipative state. We present some indications that this transition does not originate from temperature instabilities but is driven by the current. [ABSTRACT FROM AUTHOR]