When, some years ago, Lady Antonia Fraser turned her attention from the mysteries of the past to those of the present, detective fans thought they had reason to rejoice. The very intelligent and literate biographer of Mary Queen of Scots, King Charles II and Cromwell surprised us all when she created a sophisticated series of murder mysteries and a beautiful sleuth to solve them.

Jemima Shore seemed to have it all. A strapping, lithe, red-headed investigative reporter for a giant British television network, Shore was well-educated, urbane, witty and pleasantly upper crust. While on vacation or location, she had the requisite habit of stumbling over murder mysteries and applying her investigative instincts to providing a speedy solution. She was, moreover, feisty, sexy and would attack the most dangerous murderer with courage and cool.

In spite of Shore's appeal, Fraser's series has always had some rankling flaws. The names of her characters and locales make one wonder if the author is trying to spoof detective fiction or if she's really serious.

And what of Shore's studied, casual sexuality? Isn't there something a tad too predictable about her insistence on relationships without commitment (that dread word), and her willingness to take a quiet tumble with whoever steps into her bedroom?

In the first three Shore novels these problems tugged gently at the reader's credulity. In her next novel -- "Cool Repentance" -- they became more pervasive. But in "Oxford Blood," her most recent offering, Fraser's failings strain the curiosity and patience of even the most avid fan.

"Oxford Blood" is the tale of a young English aristocrat who, Shore discovers, may not have the pure blood that's supposed to course through upper-class veins. Shore inadvertently uncovers this mystery when she receives a call from a dying woman who was nurse to Lord Saffron's family. In an effort to cleanse her conscience before she meets her maker, Nurse Elsie tells Shore that the real Lord Saffron died at birth. To save his mother's sanity and the family fortune, a substitute baby was put in his place. Could it be true?

Her curiosity piqued, Shore finds the opportunity to investigate further when her boss insists she do a series that exploits the popularity of "Brideshead Revisited." What he wants is a portrait of the "Golden Lads and Girls" at Oxford -- a portrait that focuses on none other than our mystery viscount, the 20-year-old Lord Saffron.

Delighted that she has an excuse to meet him, she races down to Oxford. There she finds that a student living on Lord Saffron's floor in an old Oxford college has been found dead, an unknown assailant tries to murder the young lord, and his fiance' dies of a drug overdose. Shore soon has another assignment: When it's clear that someone is after Lord Saffron, she becomes the young lord's protector.

Just who that someone is, is, of course, the mystery she must solve. But it is so poorly plotted, so clumsily told, so peopled with dull and deficient young waifs and wastrels that only sheer willpower keeps you going.

Needless to say, Shore solves the case and all ends happily. Except, that is, Fraser's reputation as a mystery novelist. Like a comedian striving desperately for a laugh, Fraser is forcing rather than cultivating her talents. Instead of producing novel after novel, what Fraser obviously needs is a long rest. Perhaps Shore should fall madly in love, get married, have a baby and then return to sleuthing. After, that is, Fraser returns to the form of which she is a genuine master. Maybe history will give her the inspiration that is so sorely lacking in "Oxford Blood."