CBS' once maturely adult Lesley Stahl has converted into Medialand's breathless Miss Goodie Two Shoes. You may recall her at the Republican convention leaping up and down after the less than galvanic news of George Bush's selection, shouting, "Uncle Walter , I got it first! I got it first, didn't I? Wasn't I a good girl?"

A more subdued version of the reporter as ingenue reappeared at President Jim Bob's press conference, held to discuss the momentous question: Am I My Billy's Keeper or Is It Even Possible to Be My Billy's Keeper?

At the end of a number of questions distinguished only by their sour tone, repetitiousness and argumentative thrust, Miss Blondie Stahl secured the floor to ask how come if the president hsn't committed any crimes, improprieties, unethicalities or no-nos of any kind, how come he was in such a mess? Being in a mess having become prima facie evidence of political turpitude, it puzzled television journalism's darling girl that President Jim Bob could insist be wasn't guilty of something.

Overlooking the unpardonable virginity with which Miss Stahl posed the question, it is nevertheless a good one.How in hell did so much trouble come upon Jimmy?

You can't blame the Republicans. They're not responsible for the Billy-goat matter, and the willier of them, sensing that this is a case of there being less here than meets the eye, have been circumspect in thier language. It isn't they, it's the media who have slung the accusations around.

It was they who were implying the other night at the press conference that Roselynn was a sinsiter figure in the White House, like the last tsar's wife -- that German witch, they called her -- who, through he disreputable. fascination with Rasputin, brought discredit on the Supreme Autokrator himself. The comparison is as appositeas comparing Lucrezia Borgia and her relatives with the Waltons.

Heavens, there are grounds enough forattacking Jimmy Carter. Anybody who shops in a supermarket can tell Blondie and her friends as much. Then why do they persist, as they have done almost since his inauguration, to make President Jim Bob out to be a crook or, de minimis, a seazy sort with a pack of crud bag relatives?

The one area in which the conventions of journalism allow reporters to editorialize is in the crime, morals, ethics sphere. They must pretend to some neutrality of opinion when reporting on foreign affairs or unemployment, but the least hint of impropriety, and they have permission to dilate on the subject in a manner worthy of a born-again parson.

Since Watergate, these hounds have been blooded. They can't help snapping at presidential flesh. The point was nicely exemplified the other night on CBS immediately after the press conference. Reporters Fred Graham, Robert Schackney and Bob Pierpoint were doing the ritual rehash and all agreed in their various ways there was no evidence of guilt, indeed no evidence of a crime, a conclusion that would haveled a sober juror to declare "case closed."

But no such thing. After having failed to find a crime, much less culprit, they fell on President Walton for lack of leadership and poor judgment like vampires who had abstained ever so long from taking a bite, but in the end couldn't help themselves.

What the media once set out to expose they has now addicted them selves to. From Watergate to Mediagate, they're doomed to repeat and repeat this Transylvanian Tragedy. Bring back Nixon. He could help them complete the infinite media circle of the crook president covering up and conspiring, always to be exposed, impeached, and thrown out only to reappear being inaugurated again. The Media Morality Cycle, it will be called by future students of social symbolism and political ritual.

But how to break the circle? Today's big-time journalists live in an isolation comparable to that of Richard Nixon before the fall. Their judgement is poor because, always traveling together, always being together, never knowing outsiders save in the artificiality of the interview situation, all sense of proportion atrophies. That was the cause of the flap over Jerry Ford at the Republican convention. Put 11,000 reporters in a room virtually by themselves, and what else can you expect? The best wayto break the circle would be to require every practicing political journalist to spend one month a year working in the lumber mill on Walton's Mountain. While having their common sense re-educated back into them, they would come to appreciate Grandpa Watlon's wise old saying that "where there is no smoke there is no fire."