One convenient thing for a writer reviewing a Cat Power performance is that you pretty much know exactly what to expect. Singer Chan Marshall’s live onstage emotional breakdowns have become legendary for their regularity at this point and Wednesday’s sold out ’Sco performance proved no exception, at times feeling more like a therapy session than a rock show.

Marshall has a lot going for her as a singer-songwriter: a whispery, whiskey-soaked voice which would sound equally at home on Bourbon Street or Bedford Avenue, an appropriately folksy bio — the daughter of an itinerant pianist, she dropped out of her Atlanta high school to try and make it in New York — and what seems to be an uncanny ability to connect emotionally with her growing legion of fans.

On her last three albums, however, Marshall seems almost pathologically unwilling to push herself musically beyond the twangy, minimalist ballads (you could call it “sadcore” if you’re into that sort of thing) which have made her a commercial success. Despite occasionally stellar tracks such as “He War” and “Good Woman” off 2003’s You are Free and “American Flag” on 1998’s Moon Pix, the albums tend to get bogged in the monotony of simplistic arranging and so much angst.

An album of covers released in 2000 consists of little more than slowing down and stripping away the instrumentation of old favorites by the Velvet Underground and Bob Dylan until they sound just like Cat Power songs. The new song “The Greatest” off her upcoming Matador release of the same name promises more of the same to come.

Wednesday’s show started out promisingly enough with Marshall, performing unaccompanied alternately on piano and guitar, advising her fashionable crowd to sit on the ’Sco floor to counteract the venue’s embarrassingly dreadful sight lines. After only one song, Marshall started to seem uncomfortable and asked for a beer that was immediately placed in her hand by an accommodating concert board organizer. It was all downhill from here.

Marshall interrupted her performance of “The Greatest” several times to ask the ’Sco’s sound technicians for adjustments to her microphone levels. I found myself wishing she had applied this same level of perfectionism to her blocky piano voicings or even remembering her guitar chords and lyrics.

Marshall’s gradual slide into incoherent, obtuse warbling was interrupted briefly by a refreshing cover of the often-played but rarely well-played “House of the Rising Sun.” The line, “Father was a music man, you know what that means” seemed to strike a very personal note with Marshall, and the song’s evocation of old-time bawdy New Orleans managed to assume an air of poignancy even over the irritating clang of beer bottles.

Halfway through the song, however, Marshall seemed to get bored, attempted to change the tempo, but found the new arrangement beyond her abilities on the guitar and eventually just gave up. The Velvets’ “I Found a Reason,” Ray Charles’s “Hit the Road Jack” and countless numbers of her own songs were to suffer the same fate.

Marshall’s lyrics are filled with lines such as “The whole thing makes no sense, I wish I could tell a lie” and “Maybe I’m your answer, maybe I’m your living proof,” and “Why can’t I be loved for what I am?” which allude to her own authenticity.

“What you are seeing is not an act,” she seems to be telling us. “This is who I really am.” I’m not so sure.

Whenever Marshall would give up on a song and apologize for her lack of talent, the engaged crowd would only cheer more and shout words of encouragement, giving the whole event the feeling of a middle-school play where the lead actress keeps forgetting her lines.

If a performer reenacts the same onstage melodrama night after night, at what point does it become schtick? Something akin to James Brown pretending to collapse on stage. The difference is that everyone knew Brown was faking it and it was all in good fun, whereas Marshall is supposed to be too honest and serious for those theatrics.

The more disturbing possibility is that Cat Power is actually the real deal, and legions of indie-rock fans are anxious to wait in long lines and pay seven dollars to watch a deeply disturbed woman enact her emotional trauma for an audience.

The high point of the show, for me, strangely enough, was a medley of Frank Sinatra’s “Blue Moon” and Otis Redding’s “Try a Little Tenderness.” They seemed to be the one point in the show where Marshall forgot about “connecting” or “being honest” and just played two sentimental old songs for all they were worth. The songs worked beautifully with her voice and blended together seamlessly. This was the one point where it seemed that there might actually be something unique and interesting behind Marshall’s pseudo-emotional posturing and saccharine self-promotion.

By the time Marshall closed her set near one in the morning by walking off stage without really finishing the normally haunting “Werewolf” from You Are Free, I had been driven completely mad by listening to variations on the same chord progression for the past half hour. The rest of the crowd, however, seemed thoroughly engaged, leaving me with the overwhelming impression that there was something I just wasn’t getting.