intermittent failure of function

I have a function that has been in use for years without problems. Now it fails at seemingly random times.  I can run it against the same data and once it will fail, next it will succeed.

I traced it this far from the failure of a trigger failing at the line

:new.shape := UTM27_TO_WGS84_SDO(:new.easting, :new.northing, :new.zone);

2

Does anyone have experience with something so hard to nail down? I don't know where next to look, as everything compiles, is defined correctly, and sometimes runs successfully.

I'm on Oracle 12.1.0.2, Windows 2012.

Thanks.

Problem function:

create or replace FUNCTION   "UTM27_TO_WGS84_SDO" (easting IN NUMBER, northing IN NUMBER, zone IN  NUMBER)