I'm thinking that under heating will cause grainy candy. I heat my syrup to 234* when I make cream and if I stir it too soon like at 165* it comes out grainy. I heat my syrup to 244* when I make candy and stir at 175* and it comes out smooth. I'm not sure why the sugar crystallizes the way that it does at the different temperatures but 10* more does make a difference with the candy.