It seems fine until it cools off and is reheated...
Maybe when you re-heat it overcooks the chocolate. If its smooth and creamy until it is reheated the problem seems to lie there. I am not 100% on that just a thought. I know if my chocolate is over heated it gets grainy it can be very finicky.
Maybe not enough "fat", try adding some butter.
Add cold butter and mix in a food processor
I'm not sure if this applies to a chocolate sauce. But, in general with chocolate, cooling quickly causes large grains to form.
If you're cooling the sauce down quickly it will form large grains. Bring the heat down very, very slowly. Maybe even keeping in the oven at superlow and turning off the oven element. Also note that 'quick' is very relative term when working with chocolate.