Following the cool reception to Insomniac, Green Day retreated from the spotlight for a year to rest and spend time with their families. During that extended break, they decided to not worry about their supposedly lost street credibility and make an album according to their instincts, which meant more experimentation and less of their trademark punk-pop. Of course, speedy, catchy punk is at the core of the group’s sound, so there are plenty of familiar moments on the resultant album, Nimrod, but there are also new details that make the record an invigorating, if occasionally frustrating, listen. Although punk-pop is Green Day’s forte, they sound the most alive on Nimrod when they’re breaking away from their formula, whether it’s the shuffling “Hitchin’ a Ride,” the bitchy, tongue-in-cheek humor of “The Grouch,” the surging surf instrumental “Last Ride In,” the punchy, horn-driven drag-queen saga “King for a Day,” or the acoustic, string-laced ballad “Good Riddance.” It’s only when the trio confines itself to three chords that it sounds tired, but Billie Joe has such a gift for hooky, instantly memorable melodies that even these moments are enjoyable, if unremarkable. Still, Nimrod suffers from being simply too much — although it clocks in at under 50 minutes, the 18 tracks whip by at such a breakneck speed that it leaves you somewhat dazed. With a little editing, Green Day’s growth would have been put in sharper relief, and Nimrod would have been the triumphant leap forward it set out to be. As it stands, it’s a muddled but intermittently exciting record that is full of promise.