Sandy chocolate truffles

I mentioned recently that I've been making some orange truffles that I'm really happy with. Everybody at work liked them, too, but one guy took some home to his wife who is an expert truffle maker and she said the darned things are gritty! Haha! I know what she's talking about. Although I thought that batch was smoother than most, I've noticed just a slight grit to the centers.

I'm using good chocolate, Guittard's clbuttic bittersweet, so I don't think that is the problem. There could be several reasons they have a grit to them, but I'm thinking that most likely it's because I'm adding sugar to the mix to take a little edge off the bittersweet.

I'm heating up 1 1-2 cups of heavy cream, stirring in 1-4 cup of sugar, and then turning the heat down to just about nothing and adding 24 oz of chocolate. Although that might seem like a lot of cream, the higher percentage chocolate seems to require it. I add half a stick of unsalted butter, too.

I have several questions:

1. Is the sugar most likely the reason for the grit?

2. If yes, is there a better way to sweeten it so that I don't damage the truffle texture?

3. Is there some other reason it might be getting gritty? I have mentioned that because I like the centers soft, I have to refrigerate it quite a while (usually overnight) to thicken it, and then I have to put it in the freezer for about half an hour (but the last time they might have been in there for several hours) after I ball it up. Could this be making it grit?

4. It wouldn't be because of the orange liquer I add to it, could it? I wouldn't think so.

5. I'm fairly careful about the temperature that I bring the chocolate to. I don't think that's causing the grit but I guess it could. I heat the cream to almost boiling and then turn it down to practically nothing, and I stir the chocolate in it constantly.

Thank you! Michael