For the first few seasons that we used our Gen 3 monitor in the combine we had 150 acre fields registering as 225,000 acres.  We were exporting GPS out of the CaseIH NAVII controller.  I later got GPS directly out of the 372 Port B and didn't have a problem.  Last year I went back to the NAVII and did not have an issue.  I was told it has to do with a corrupted or inconsistent NEMA data string.