The Laws of Gravity Seem to Tug at Letterman : Television: What’s the problem with ‘Late Show’? Take a look at a night-by-night comparison with Leno.

Happy anniversary, Dave.

Or, umm, is it a happy anniversary? Two years ago Wednesday night, “Late Show With David Letterman� went into orbit, and what an orbit it was. But laws of gravity being what they are, “Late Show� has felt an inexorable tug earthward in recent months.

Problems? Take your pick--the poor CBS prime-time schedule, stronger competition, loss of key staffers. You’ve heard ‘em all. But, truth be told, something else is amiss. Our avowed purpose is to explain what that “something else� is.

We watched tapes of a full week (Aug. 14-18) of “Late Show� and “The Tonight Show With Jay Leno,� hoping to catch a fleeting glimpse of the Problem. Why this particular week? (A) It was the last time Leno and Letterman went head-to-head with live shows, and (B) it’s the dead of summer, and big-name guests are hiding out, which means that the hosts have to do all the heavy lifting.

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Night 1: A “Tonight� fixture is elaborate shtick, and we get a double dose. Leno goofs with Newt Gingrich, who muses about becoming host of “Tonight.� There’s also a remote to a beach house, where a bunch of California Ken-and-Barbies are lounging about. Kennedy of MTV, in fact, roasts Barbie dolls in a microwave, while a 300-pound guy jumps in a pool with an airline seat to see if it works as a flotation device. Yes, the bit is endless. But so is the interview with the first guest, David Spade, who is perhaps the dullest talk guest alive.

Ahhh . . . no, he’s not: Letterman’s first guest, Kelsey Grammer, is. Grammer, divorced twice, talks about his new girlfriend, and the conversation is rather sad. For shtick, Letterman has Rupert Jee of the Hello Deli touch meat. It’s amusing the first time, but by the the fourth time (the smoked turkey), it’s downright gross. This is a key Letterman problem: He sometimes beats an initially good joke into the ground. Even jokes at the expense of Dick Assman--the gas-pump guy from Canada whose name has made him a Letterman staple--are getting tired.

Winner: A tie.

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Night 2: We have located what we think is an Achilles’ heel of “Late Show.� Letterman loves shtick that involves guests; tonight he plays something called “Will You Operate Our Camera?� A young Venezuelan woman (cornered in the pizzeria next door) gets to operate the hand-held camera. She carries it around, and the audience roars. But viewers at home go to sleep. The bit’s dull and pointless. Here’s the problem: Average folk do not usually make funny TV (unless, of course, this was “America’s Funniest Home Videos,� which it is not). But often Letterman hands the show over to a pedestrian or audience member. By contrast, Leno produces everything. He also has the night’s best monologue line: “Mark Fuhrman’s writing a new book--it’s called ‘Mein Kampf.’ � (Leno, a superb stand-up, often beats Letterman in this crucial element--he pumps out an average of about 18 jokes per monologue to Letterman’s eight.)

Winner: “Tonight.�

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Night 3: Leno has a double coup: Kevin Costner and Dee Dee Myers, making her second talk-show appearance after getting busted for drunk driving. Costner, badly bruised from bad press for “Waterworld,� is genial and amusing. Myers makes a plea for people not to drink and drive.

Conversely, an awful night for Letterman. He seems crankier than usual (you’d be, too--he says it took him two hours to get to work). His guest, Isabella Rossellini, has virtually nothing to say, and Letterman just dies. Example? He jokes about her name, EEEEsabella RossaLLEEEEni.

Winner: “Tonight.�

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Night 4: A terrific night for Dave, finally. He’s really on, and the show hums. Why? First, the monologue is longer (13 jokes) and contains more than a couple of good jokes (Bill Bradley’s quitting the Senate, he tells reporters, because he got tired of Sen. Bob Packwood hitting on him). The Top 10 List (“Surprises in Mark Fuhrman’s Tapes�) works (on nights it doesn’t, the show gets dragged down). Letterman also seems to be enjoying himself: After a fly lands on his face, he shouts, “IT’S A BAT!!!� Also, that old Letterman guest standby, David Brenner, is, as usual, amusing. Meanwhile, Leno’s dying. His guest, Sinbad, just doesn’t hit his mark.

Winner: “Late Show.�

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Night 5: What will Leno do when the O.J. Simpson trial ends? It seems to provide half the fodder for his jokes. He gets in a good one tonight. He talks about how Judge Lance Ito met his wife “at a murder scene. Here’s my question--who hits on a woman at a murder scene?� First guest, Antonio Banderas, is charming.

Over at “Late Show,� Letterman gives his longtime announcer, Bill Wendell, a gracious send-off and introduces a new bit, “Small Town Commercials,� which is terrific. One really bad car commercial features some guy doing a “Top 10�; Letterman says he thinks the fellow “was asked to host next year’s Academy Awards.� But the best bit of all: The world finally gets to see Dick Assman in the flesh!

Winner: A tie.

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Conclusions: Letterman: Happy to report that he’s still a fine late-night guy, but there is a troubling lack of consistency. Some nights he is simply not there--no pop, no enthusiasm. Is something distracting him? We hear his new house in Westchester, N.Y., has been gutted for renovation. Maybe he’s fretting about the choice of wallpaper.

Leno: The talk guy who will never get any critical respect is--surprise--putting out a very durable show each night. “Tonight� is the Ford pickup of late night--not much glamour but it gets the job done.