If the grinding is intermittant it is most likely the nature of the lining material. You really need to pull the drums off for an inspection to see if there is abnormal wear.
If the wear looks normal and even, there isn't much you can do, but it helps to clean up any brake dust and make sure that the contact points where the shoe rubs against the backing plate is properly lubricated. It also may help to sand the lining face and drum contact face lightly to break up any glaze that may have formed.
