Another guy in his position-really high-paying new job on the horizon, liberation from the frustrating, really high-paying old one right around the corner-would just be going through the motions, watching the clock, waiting for a cab, snoozing. Not Dave Letterman, Our TV Friend, as he has taken to referring to himself during the final days of his NBC employment. You’d think Letterman would be acting bored or impatient, but you’ve got to remember, Dave has already done the Bored Thing. When Jay ”I Used to Be a Contender” Leno gained possession of The Tonight Show (”took control” being too strong a phrase for Leno’s still-slippery grasp on Johnny Carson’s heritage), Letterman, now 46, looked around his Late Night kingdom and immediately became restless, dissatisfied, angry. (And thin, pencil-thin. What’s he down to, about 125 now? If he doesn’t start knocking back some chips and a few brews once in a while, Dave’s looking at the leading role in The Richard Carpenter Story.) Yes, in the wake of NBC’s Big Mistake, Letterman succumbed to boredom, even as he was careful to turn his dark mood into lotsa laffs. However, once Letterman permitted agent Mike Ovitz to parlay him into The Hottest Guy in Show Business this past winter, and once CBS paid him the respect (and the $14 million a year) he felt he deserved, Letterman gained a serene happiness, a cheerful assurance he’d never had before. The result is what we’re watching now, as Letterman nears the end of his NBC era on June 25: a relaxed, confident fellow who, armed only with immense power, wealth, and his own private mantra (”Buttafuoco”), is going out a mensch. Or, as he put it recently, ”You ain’t dealin’ with no monkey here.” Consider some of the outstanding moments Letterman has presided over in recent weeks: *He paid his respects to Cheers, noting that his favorite character was ”Sonny, who tends bar.” *He has introduced no fewer than three new occasional characters: Peaboy (a green-suited maniac who runs up and down the studio aisles throwing frozen peas at the audience); Peggy the Foul-Mouthed Chambermaid (her obscenities are bleeped, but easily lip-readable); and, best of all, William, the NBC Page With the Fake British Accent (it’s cockney: ”’Allo, Dive!”). *He conducted a classic passive-aggressive interview with Sharon Stone, during which the actress followed her dismaying basic instinct and made a series of coyly lewd comments to Letterman for two full segments. This prompted a certain exasperation from the host. When Stone stumbled verbally and said, ”I can’t put a sentence together-thank God I can take my clothes off, eh?” Letterman looked out at the audience and said, ”It’s good to see somebody call a spade a spade, isn’t it?” *And finally, while chatting amiably with Will ”Fresh Prince” Smith, costar of the new movie Made in America, he asked a question much on every citizen’s mind: ”What’s the deal with Whoopi and Ted? And if there is a deal, how the hell did that happen?” So, as Letterman leaves the air until the Aug. 30 debut of his Late Show on CBS, let us conclude with the host’s own sage advice for the steamy summer ahead: ”Every time you go out in the yard, the second you get back in the house, strip yourself naked and inspect yourself for ticks. You’ll thank me on Labor Day.” See you before then, Dave.