Thank you Gerhard..
This makes it even stranger. If the unit has a date 1970 at each boot up, the PC uses current date and the .nav files (in Windows Explorer view) are dated 2097 and 2098

I have tried 3 different rx´s all with the same dates, all working well - so I settle with this.
I am not sure I understand the GPS part of your answer.
Are you saying that it is normal for the Time(s) column in the .nav file to stop showing seconds as soon as the GPS are locked on to satellites?
When ignoring frames without valid gps position all the sequenced time intervals are removed giving only a first cell of 0:00 before the xxxxxxxx:xx format appears.
Any idea what this Time(s) column data means: 21283867:44 ?
So no problem using my RX´s then, good to hear.
Best regards
Jo Erlend