  We report measurements on magnetization reversal in the Fe$_8$ molecular
magnet using fast pulsed magnetic fields of 1.5 kT/s 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 $\sim$ 4 K. We also observe shifts of the resonance fields
in 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.
