Reading this again, It looks like the revs slow down but the motor doesn't stall ( you say it runs in 'derate' mode).
At that point did you turn off the ignition and try to restart.? Can you confirm that your engine alarm sounder works.
It will help to know if the engine just dies or instead slows down.
Also reconfirm that when it does whatever it does, you turned off the ignition and restarted the motor with no difficulty.
When you had the problem and restarted, did the problem reoccur after a period of time cruising?