The good news is that the epic-sized rock documentary “Metallica: Some Kind of Monster” is, in spots, as amusing as “This Is Spinal Tap.” The bad news is that it’s not supposed to be a comedy. The 140-minute opus from co-directors Joe Berlinger and Bruce Sinofsky covers a couple of difficult years in the history of the popular heavy metal band, during which the guys struggle against their personal demons, internal rivalries and artistic hangups to try to produce a new album. In 2001 their bassist Jason Newsted had left, and the remaining three members–James Hetfield, Lars Ulrich and Kirk Hammett–attempt to regroup and, with producer Bob Rock filling in for Newsted, go into the studio to create a discful of new songs. Unfortunately, tension between band founders Hetfield (guitar) and Ulrich (drums) leads them to add another member to their troupe–a therapist named Phil Towle, hired to help them work through their problems. (The soft-spoken, unobtrusive Hammett, meanwhile, pretty much stays in the background.) But things deteriorate when Hetfield goes off to rehab, returning months later with new rules that set off Ulrich, who’s also gaining greater notoriety for his very public attack on Napster as theft–a position that earns him the scorn of a great many fans. But ultimately the trio does produce an album called “St. Anger,” adding a new permanent bassist, Robert Trujillo, in the process, and are feted in a tribute program on MTV.

It could be said that the titles of Berlinger and Sinofsky’s two previous documentaries, “Brother’s Keeper” and “Paradise Lost,” apply equally well to this picture. Much of the footage depicts Metallica as a kind of dysfunctional musical family whose members are less solicitous of each other than the backwoods New Yorkers of the first film, and the unhappy state of the band certainly is a sad decline from their early glory days. But what’s most notable about the directors’ long, repetitive treatment of this period in Metallica’s history is what comes across as an almost ludicrous sense of earnestness and self-importance. What becomes abundantly clear is that the Metallica trio take themselves very seriously indeed–and also that they whine endlessly about their troubles even as Ulrich, for example, is shown blithely selling off his huge (and immensely valuable) art collection. Their extravagant breast-beating is likely to irk even their fans, most of whom are probably a lot worse off, and make them guffaw at the fellows’ extraordinary degree of self-absorption. Under the circumstances it’s a very pleasant interruption when Ulrich’s father, an emaciated, bearded fellow, shows up to offer stinging criticisms of his son’s recent musical efforts. You feel that Lars deserves being brought down a couple of pegs.

Even funnier is the band’s involvement with Towle. This New Agey fellow’s presence seems more than a little ridiculous from the start, but when he becomes increasingly intrusive the band’s reaction is predictably negative. By the end they’re furtively conspiring to get rid of him, while he’s come to consider himself virtually indispensable and planning to stay on with them indefinitely. For all the emphasis on the interaction among the musicians, it’s this rather loony psychological detour that gives the picture a frequently hilarious Entertainment-TV spin, as though it were a hopelessly elongated episode of “Behind the Music.”

Simply from the documentary standpoint “Metallica” has a lot going for it: Berlinger and Sinofsky are clever guys, and they cagily gain access to often embarrassing incidents and are adept at setting up dramatic moments, as when the remaining band members visit the debut of Newsted’s new group or confront guitarist Dave Mustaine, whom Hetfield and Ulrich dumped years earlier. But in the end all the fanatical attention and crocodile tears make the group’s difficulties seem like small potatoes inflated beyond all reason; before the two hour mark, you may well feel like shouting at the screen, “You think you’ve got problems?” By the end many viewers might think the subtitle should really be “Some Kind of Joke.”