For something different--not French food--try the rooftop restaurant at the top of the musee du monde arabe (accents missing/spelling?) if it's open. Nice views and last summer it seemed popular among the natives. Or, to echo a suggestion posted above, go to one of the top bistros or (my idea) one of the famous old restaurants in Montparnasse.

Michelin used to have a list of restaurants in Paris open in August, as well as Sunday, late night, etc. Even an old version of the red guide might be a helpful starting point if you can get your hands on one.

In general, some of the restaurants in hotels are likely to be open. I've had good meals in le Bristol. Les Ambassadeurs (spelling?) in Hotel de Crillon at one time had two or three stars and was open on Sunday nights, which was very useful to me several times. Paris (VIII, in hotel also I believe) might be open, although I've never managed to get a reservation there.

I think l'arpege used to be better. In fact, the list posted above shows how many restaurants have fallen in recent years, including some of my old favorites like Jacques Cagna (definitely not open in August). I tried Helene (accents) Darrouze once, when it was new and very highly rated, but I was disappointed, even though I didn't expect it to be at all like the family's old place in Auch.