Welcome. The only disease worse than the green machine disease is the Unimog disease.
The intermittent is probably the oil pressure sensor. It's easy to check. On the OM-352 engine the oil pressure sensor is along the left side (drivers side) of the engine block, about halfway back. It's below the dipstick for the power steering fluid and in front of the starter. Sit in the drivers seat with the doghouse cover off and you can see and reach it.
From the screw contact on the end of the sensor measure the resistance to ground (the block) and see it's about 20 Ohms. You will probably find much higher than that if the sensor is bad. Occasionally the sensor threads lose electrical contact with the block due to sealant, so use an open end wrench and tighten the sensor, screwing and unscrewing to make electrical contact. Check resistance again. If still much higher than 20 Ohms then the sensor is probably bad. I'd suggest removing it completely and cleaning the contacts, then check it again wit the meter.
Have you joined the online Unimog groups? Lots to learn there. Look up Benzworld/Unimog at
http://www.benzworld.org/forums/unimog/
and the Moglist at
http://www.tx4x4.com/index.php?page=uml
One source of the oil pressure sensor is
http://www.expedition-imports.com/
Another source:
http://www.eurotruck-importers.com/,
And
http://www.eurotech-services.com/index.html
You can also go to Rocky Mountain Moggers webpage, check the links for other vendors:
http://www.rockymountainmoggers.com/links.html
Bob
(I do not have an FLU-419 but many systems are the same as a 406 Unimog when you are searching for stuff.)