A ROCK STAR'S ability to sustain a loyal following for more than a decade is rare. The list of those who survived the '60s -- and, most likely the '70s -- could be catalogued on one side of an index card.

But while fame has the tendency to be meteoric, some performers have staved off extinction by resorting to periodic face-lifts in order ot rejuvenate their popularity and inspire new direction.

The Beatles are perhaps the best example of this cultural metamorphosis. Hardly obsolete when the group disbanded, their music was perpetuated by the musicians' solo careers and, in some cases, enhanced by fresh outlooks. Paul McCartney's latest string of hits is certainly testimony to his versatility; Ringo Starr stumbled upon previously uncultivated, if somewhat overstated, talent; and John Lennon's self-imposed exile is rumored to be nearing a fruitful end.

Only George Harrison's star has oscillated with each new album. In 1970, his three-record set, "All Things Must Pass," signaled the emergence of Harrison as a songwriter to be reckoned with. The material on that album was consistently strong and, in some instances, exceptional. His "Concert for Bangladesh" in 1971 maintained the momentum and placed him in good company with the likes of Bob Dyland and Eric Clapton. But the next four albums were dreadful mistakes and were not at all well received.

Harrison's new release, entitled simply George Harrison (Dark Horse, DHK 3255) marks a renaissance of sorts.On it, the erstwhile Beatle once again proves a first-rate composer in the tradition of "If I Needed Someone" and "Something." There is a conscious sense of structure which was absent on the previous albums: Harrison not only arrives at interesting melody lines, but he infuses them with protean guitar riffs to supplement alternating choruses and unavoidable repetition. His sparse, often puerile lyrics, however, are hardly compelling.

Still, there are several fine songs on George Harrison : "Love Comes to Everyone" is a jaunty opening track propelled by Eric Clapton's racy lead guitar and a standout performance on the synthesizer by Stevie Winwood; "Not Guilty" features Harrison's classic instrumental accentlurking behind a bluesy ballad; "Here Comes the Moon" is an equally engaging send-up of the singer's earlier stellar hit; and "Blow Away" and "Faster" are both bright and imaginative tunes which should find wide appeal among top 40 audiences.

There is a tendency to overlook Harrison's vocals when giving this album a cursory listening. For whatever reason, they come off as a minor ingredient, langorous and unemotional. But, upon reexamination, it is a combination of those very inflections which makes George Harrison's style so instantly recognizable. His giddy phrasing, the manner in which his voice trills from one end of the octave to another, his lilting falsettos are distinctive effects which add to the album's uniqueness and the artist's individuality -- all, of which, stand to insure his longevity for many years to come.

Legendary pasts, however, are not criteria enough to preserve a fading star or even warrant a revival. Some groups trying to stage a comeback are finding that the competition is too fierce and the public's expectaion exceedingly high.

In 1968, when the Byrds -- then Roger McGuinn, Gene Clark, Michael Clark, Chris Hillman and David Crosby -- began to undergo a series of personnel changes, serveral members formed splinter groups of their own, although only Crosby enjoyed any widespread commercial popularity. Because of their heritage, record companies were willing to take a chance that one might connect with the Byrd's previous success formula.

The most recent concoction is a joint effort called McGuinn, Clark, and Hillman (Capitol, SW-11910) which falls far short of its intended mark. It is a bland, middle-of-the-road album, addressing neither punk-rock insurgents nor the folk-rock alumni of the late-1960s. It is background music, not recorded, but Manufactured for the "mellow" market which has conquered FM radio. The instrumentation, as well, is incidental.

Perhaps this band would have been better sutied adapting familiar hits, much as Linda Ronstadt and the Pointer Sisters have done. Their vocies ar quite pleasant and blend well together. The material, however, is vacuous and requires no discussion, except for Roger McGuinn's plaintive "Don't You Write Her Off," which brings to mind his three wonderful solo albums.