The named range is defined in an excel file (used as an template file) that gets loaded at statup, and then I insert rows within the range. It appears to me that the range rowCount doesn't change after insertion. eg.
Range rng = excel.Worksheets.GetRangeByName("dataGroup");
int rowCount = rng.RowCount;
Debug.Assert(rowCount == rng.RowCount + 100); // this seems to fail.
rng[rng.RowCount + 99].PutValue("test"); // this will throw out of range exception