In a singles bar, a young advertising exec is explaining the power of subliminal TV ads to a disbelieving young woman. As he talks to her, he adroitly uses flashcards to suggest a developing relationship for the evening.

It's very funny, indeed, and a promising opening for "An Evening Without Liza Minnelli," the new comedy revue at the New Playwrights' Theatre through Dec. 31.

Alas, except for two or three other sketches, the rest of the material in this 50-minute show is thin. When it was over, the audience hesitated in leaving, and one man observed: "The indecision over whether its intermission or the end says something about the show."

Tim Grundmann, whose comedy range extends from biting satire to antic humor, collaborated on the revue with two stylish performers, Jan Frederick Shiffman and Dana Vance.

It's a three-way collaboration that produced some outrageous moments in last summer's "Grownups." This time "Evening" is only fitfully inspired. i

In the last 3 1/2 years, Grundmann has submitted seven musical revues to the New Playwrights' Theatre. That is a prolific outburst, but the new show suggests that it might be wise to take more time to refine sketches into a solid revue.

But "Evening" does have its moments if they are too few. There is a zany sketch in which Vance narrates a tragic love affair with a cookbook that becomes overdue, and must be returned to the library. And there's a catchy tune, "I'm a Jogger," to close the show.