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I have a RostockMax V3.2 with Duet board and SE300 dual feed hotend. It was purchased 3/24/18.

I have a random issue where the hotend heater will have a fault for temperature out of range. I haven't been able to tell if the temperature abruptly peaks too high to create the fault or if it drops down which causes the fault. The graph just shows a steady decline.

I have replaced the entire SE300 hot end assembly without a change. Both assemblies (board, heater, sensor, dual filament feed, etc. everything except the whip cable) have the same issue on occasion.

I have replaced the config file with several different files that Steve sent over the years, and that doesn't solve the problem.

I tuned the heater loop using the procedure that Steve supplied. That sometimes appeared to help for a while, but did not resolve the problem.

I have secured the whip so that it cannot flex the connectors on the hotend, and that doesn't seem to make a difference.

The firmware is still the same as Steve installed last spring, which worked fine for several months before the problem started to occur.

I thought it might have been a Cura setting, but it doesn't always fail at the same point in a print file.

So I'm down to several possibilities that I can think of:
The Duet card is occasionally failing. I had one print that I sliced for an Orion and accidentally sent it to the Rostock. It failed from the start and the resliced file ran correctly, but it may have caused a problem with the Duet, or the Duet may have developed a problem due to age.
The whip may have an internal problem or a poor connector. It is one that I did rewire when I upgraded to the SE300 from the original SE280.

Do either of these seem likely? Are there any other possibilities that you can think of for me to check?

Update:

After I posted the above, heppster's post on a 100% fan shutdown struck me as being a similar symptom so I checked out how the tool fan speed affected temperature control. Heppster noted that 50% fan speed eliminated the problem. I checked and at 100% I had a variation of +- 4 degrees, where as at 42% it was +- 0.6 degrees. I've only run one print, but it was much better quality and ran to completion.

I will do more checking and post results in the near future.

After changing the fan speed setting in Cura to 42%, I have run over 30 hours of printing with no faults. So thanks to Hebbster for locating the cause.

I'm not into software much anymore, but I suspect that the temperature tuning routine isn't adequately compensating for fan speed. While running it I saw a considerable sine wave generated that spanned up to 12° degrees. By manually setting the fan speed to 42% in my case, the variation was observed to be less than 0.7° and typically less than 0.4°. The consistency of my prints has improved, and no faults! Problem solved.