Everyone is trying to rollick in "The Rover," the Restoration comedy at the Folger Theater Group. ("Rollicking" is always used for "fun" in disussing Restoration drama, just as "bawdy" is for "dirty.") The adapter and director, Michael Diamond, has crammed almost continual activity into the production, keeping a lively cast busily alternating between sword play and crotch jokes. The choice of the play, an obscure 1677 work by a woman, Aphra Behn, is odd and exciting, indicating that the regime of John Neville-Andrews, the theater's new artistic director, will be imaginative. Everyone who cares about the Folger, where the standards are so high that even the failures are interesting, must be ready to rollick away if it would help. The theater has had financial troubles and an unsuccessful opening production of the season. When the theater did such a good job last year on "Wild Oats," which was no classic, either, the idea of culling enjoyable material from the forgotten works of past centuries seemed to promise a treasury of untapped theatrical delight. And yet "The Rover," try as everyone will, cannot be said to be a rollicking comedy. It is just a dumb romp, like any mindless modern script about a bunch of people looking for quick sex and money. And while literary age usually confers some charm (which is why the special vocabulary is used), it is, in this case, minimal. The cast, many of them Folger newcomers, cannot -- with the exception of Sherry Skinker as a virtuous heroine beset by troubles -- disguise the thinness of these unappealing characters. The effort is there, but not that fresh-faced, comic enthusiasm that can sometimes make the stereotypes of the past endearing to the present. THE ROVER -- At the Folger through February 21.