Hi and welcome to Knitting Help!
I think this is the pattern: http://www.ravelry.com/patterns/libr...nce-scarf-wrap
The pattern for rows 2 and 3 adds up correctly and you shouldn't have an extra stitch after row 3 if all the rows are 105. I suspect that you're right and its something about the yarn over (or yrn or yf) that might be causing the problem. Put stitch markers on row 2 to mark off the repeats (8sts). Make sure there aren't extra loops over the needle in any repeat. It's not a consistent mistake, just something that happens once.
Row 2: Yrn, P2tog, P6, *P2tog, yrn, P6*, P1 tbl;
Row 3: Yfwd, K2tog tbl, *K4, (yfwd, K2tog tbl) twice *, K7.