MP3 export problem

hi everyone
i’m using and imac with Yosemit and cubase element 8 with the mp3 encoder.
when i’m exporting a 1h file long the export cut me 6 sec.
i check every possibilities that i know.
the right and left cursor
without any plug in

i chack check in regular wav 44100hz 16 bit regular format ans it worked fine 60 mn precisly
but in mp3 59.54 mn.

please anyone have an idea??