We have taken it to the Fiat main dealer garage that deals with commercials and motorhomes. They could not find a fault or code when they had it on their diagnostic machine. We had a part replaced they thought may be causing the problem at a cost of about £600, but did not solve the problem. It has been to another garage as well and they cannot see a code neither, so no one can diagnose the cause, only suggesting a couple of other parts are replaced that may solve the problem but not guaranteed to, at a cost of between £1200 to £1800.
You don't say what parts have already been done? Or the other parts they are recommending!
But all they are doing is guessing and it's your money they want to waste.
You can watch this happen so many times on YouTube, people spending £5/6000 and 3 main dealers later. Try watching Pine Hollow Auto Diagnostics, it's excellent, so many cases like yours. So don't be a mug and keep your money until you get an actual accurate diagnosis.
So how to get a diagnosis from what you have said :-
The only way that anyone will be able to give you an accurate guide, is for them to be connected to your ecu (engine control unit) ie the main computer monitoring live data with the scanner when the fault is occurring. They will have to be sat with you in the vehicle whilst it's being driven and checking all the live data parameters.
The fact that you can turn the ignition off and back on and the light goes off(reboot reset), suggests a non critical fault that hasn't developed into a more serious issue with a dtc being generated.
So you could
1 wait until it develops into a more significant fault, but you say you have been using it for a few years already. Not likely to happen.
2 get yourself a good scanner and learn how to use it, then watch things yourself as you drive.
3 find a local guy who is skilled enough and willing to get in the ducato with you at the drop of a hat, when it occurs.
Do not waste your time and money guessing, buying parts and fitting them 95% chance you will end up £1000's worse off and still have the problem.
Intermittent faults are notoriously difficult to diagnose.