BY JIM DEROGATIS POP MUSIC CRITIC

It was one of the good ones.

As the leader of the Replacements, Paul Westerberg was notorious for delivering a great show followed by a miserable one, often on successive nights. As a solo artist, he settled for a sort of bland professionalism.

Appearing alone to deliver 18 songs at an in-store performance in Chicago on Thursday, the cult-favorite singer-songwriter mixed heartfelt emotion, rock spontaneity and cheerful goofiness in equal measures, rekindling a spark that has long been missing in his work as he promoted his new double album, "Stereo."

This may have been Westerberg's only appearance here in support of the disc. Claiming exhaustion, he flew home to Minneapolis after the show, canceling the two remaining store performances in other cities. A few days earlier, he'd cut short a San Francisco appearance when he jumped into the crowd to slap a heckler.

"We've hired security now--for your protection, not mine," he joked on Thursday.

Switching between several acoustic and electric guitars, Westerberg sampled songs new and old, including the Replacements' classics "I Will Dare" and "Alex Chilton." He covered Bob Dylan and the Plimsouls, smiled at shouted requests and laughed when he flubbed the words to his own songs, relying on fans to sing forgotten choruses or hold up the lyric sheets.

When a couple requested "Within Your Reach," their wedding song, he jumped atop a display table piled high with books and sang it directly to them, sans microphone, as they shared a dance.

There were certainly dull stretches. Westerberg needs a spirited band (not just the accomplished session players of his last solo jaunts) to spur him to his very best. But the waves of emotion that swept through the several hundred fans when he played his strongest material were palpable, and they stood as a poignant reminder of what made him the most lauded songwriter of the indie-rock '80s.

The only downside to this otherwise memorable gig was the weaselly behavior of its host, the Virgin Megastore. While Westerberg's label, Vagrant Records, the Chicago press and radio sponsor WXRT-FM (93.1) all promoted the gig as a free appearance, the retailer imposed its own alternate rules, requiring fans to buy "Stereo" at a typically lofty price even if they already owned a copy, and turning away any who protested that policy.