Olney's Lesions might result from high doses of dissociatives, but they have not been demonstrated in humans.
http://en.wikipedia.org/wiki/Olney's_lesions

So 300mg of DXM twice a month shouldn't cause much of a problem. I've known people to ingest upwards of 400mg daily for weeks at a time and the result was a sort of delerium that faded with abstinence.