We report measurements on magnetization reversal in the Fe8 molecular magnet using fast pulsed magnetic fields of 1.5 kT s-1 and in the temperature range of 0.6-4.1 K. We observe and analyze the temperature dependence of the reversal process, which involves in some cases several resonances. Our experiments allow observation of resonant quantum tunneling of magnetization up to a temperature of ∼4 K. We also observe shifts in the maxima of the relaxation within each resonance field with temperature that suggest the emergence of a thermal instability-a combination of spin reversal and self-heating that may result in a magnetic deflagration process. The results are mainly understood in the framework of thermally-activated quantum tunneling transitions in combination with emergence of a thermal instability. © 2015 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.