I have been having this code lately too and I have yet to get to the bottom of it. I did find it on google and it has something to do with misfire but I can't remember what. Most of the OBD2 codes are universal and standardized between makes, but 1399 is a manufacturer specific code...i.e. P1399 for a Honda is different than P1399 for a Dodge.
On an S2000 forum I read of ppl getting this code while having problems with injectors, but this didn't seem to be my problem, so I dunno?