I had this happen to me on my 318, ended up being the voltage regulator which was allowing too much voltage to the battery 16 -18 volts.
I connected a volt meter to the battery when I was running the unit, monitoring the voltage continuously prior to engine shut down, when the engine shut down from over voltage lets say 17 continuous volts, then after shutdown voltage would drop to 13.84 volts the automatic shutdown device would reset itself & I could start the engine again.
Give it a try to check you charging system out, this could also be your problem.
I connected a volt meter to the battery when I was running the unit, monitoring the voltage continuously prior to engine shut down, when the engine shut down from over voltage lets say 17 continuous volts, then after shutdown voltage would drop to 13.84 volts the automatic shutdown device would reset itself & I could start the engine again.
Give it a try to check you charging system out, this could also be your problem.