Zumo 390 issue

[airmav]

Hello everyone,
i am from Greece so i apologise in advance for my english language.
i ve got in my posession for the last 5 years a zumo390 device installed to my motorbike with its oem base with cable direct from the battery. i have no issue with the power but at my last trip i have a serious problem with continious restarts.
i had upload the maps which i create in the ITN programme (as i used to) and at unsuspecting time the device started to restart many times. it is not power issue as i checked it,and the thing is that i realise that it did it when either it lost the signal from the satelite, either when i missed a turn.
i tried not to set a trip upload, just to put a direcet address and the issue was still there.
at some time i checked that in the screen appear a message like''system file...missing'' or something like that.
the issue was appear in three days several times but there were times that the device worked properly for long time.
as i aproach today the Greek custom service they said tha the device i no longer supported and they can't do anything. before i search for a new device is there any suggestions/recommendations?
i forgot to report that i have done all the updates avaliable in Garmin express app

[QuantumQuartz]

Have you tried a factory reset? If that doesn't work, since your device isn't supported anymore, looking into a newer model might be your best bet for stability and updated features.