‘MAGNUM’: A NEW NIGHT AT THE END OF THE TUBE

After suffering quietly for two seasons as its audience plummeted by 33%, “Magnum, P.I.” finally is getting a chance to crawl out from under the Cosby Crunch.

For the first time since “The Cosby Show” debuted in September, 1984, and turned into television’s most popular program, CBS is pulling Tom Selleck’s Hawaii-set private-eye series out of direct competition with the NBC comedy.

As it moves into reruns, “Magnum, P.I.,” the dethroned Thursday-night ratings champion, also moves this week to a new time slot. It premieres Saturday with a two-hour episode at 9 p.m. and thereafter will be seen Saturdays at 10 p.m. opposite “Remington Steel” on NBC and “The Love Boat” on ABC.

“We’re elated,” says Charles Floyd Johnson, supervising producer of the six-year-old drama series. “We’ve been under the hammer of the Cosby phenomenon for an entire year. Under the circumstances we’ve done pretty well, but what can you do against an immovable force?”

Not much. During the 1983-84 season, “Magnum, P.I.” ranked as the sixth-most-watched series on television, averaging an audience of more than 18.7 million households each week, according to ratings figures from the A.C. Nielsen Co. Last season, with the competition from “The Cosby Show” and “Family Ties,” “Magnum” fell to 15th, with an average audience of 16.2 million homes.

This season, with the surging “Cosby” series drawing half of all the people watching TV at 8 p.m. Thursdays, “Magnum” has fallen to 48th, with about 12.6 million homes tuning in each week. “The Cosby Show” is averaging more than 29 million homes weekly.

“Of course it’s been frustrating,” “Magnum” writer-producer Chris Abbott-Fish says of the audience defection. “You want to say, ‘Wait! Wait! We’re doing a really great job! Why aren’t more people watching?’ ”

John Hillerman, the veteran actor who portrays major-domo Higgins on the show, expresses much the same feeling, acknowledging that even though he hasn’t noticed any decline in fan reaction to him or the show, the dwindling Nielsen count “has been frustrating in the sense of doing excellent work and not getting the ratings you’d like.”

“I think it will be good,” he said of the move to Saturdays. Like his colleagues on the show, he hopes that viewers who chose “Cosby” over “Magnum” will now choose to watch both.

CBS apparently believes they will. The network already has ordered 13 more episodes of “Magnum, P.I.” for next season, reports Harvey Shephard, senior vice president in charge of programming. “I just feel that if we were to leave the show where it is, it would allow further (ratings) erosion to take place. By moving it, we may get another season or two out of it,” he explained in an interview.

Their happiness at the time change notwithstanding, the “Magnum” staff seems to have taken the ratings plunge with relatively good grace. One reason: the high regard in which the family-oriented “Cosby Show” is held. Cosby displays in his dressing room a letter Selleck wrote congratulating him on the show’s “well-deserved” success.

There have been other consolations as well, producer Johnson says. He notes that the series already has enjoyed a healthy run on the network and has received critical plaudits, including an Emmy for Selleck. In addition, he says that the producers have continued to make the show they wanted to make and remain proud of their work, even if fewer people are seeing it.

And smaller though they may be, the ratings could have been worse, Johnson says. The shows that ABC has thrown into that time period, for example--”The Fall Guy,” “Shadow Chasers,” “Ripley’s Believe It or Not”--have frequently drawn less than 5 million households a week and rank among the season’s lowest-rated programs.

“Our ratings weren’t anything to crow about, but we didn’t wind up being the outcast,” Johnson observes. “Your ego got a little bruised but it wasn’t desperation time.”

Indeed, Johnson’s fear now is that the switch to Saturdays will only be temporary--that when CBS executives sit down over the next two weeks to solidify the network’s fall schedule, they will move “Magnum” back to Thursdays at 8 p.m. on the theory that no other show is likely to do as well against “The Cosby Show.”

That’s not completely out of the realm of possibility, acknowledges CBS’ Shephard. “But the odds are long that it’s going to be up against ‘Cosby’ again,” he said. “We have to try something else there.”

For now it will be reruns of “Simon & Simon.” If that detective series, which has been following “Magnum, P.I.” at 9, can do as well as “Magnum” did at 8, it will probably stay there next season.

And if “Magnum” gets a permanent berth anywhere other than opposite “The Cosby Show,” “we’ll be jumping up and down with joy,” Johnson says.