WHEN a poet as partial to a pun as Paul Muldoon goes horse, you can't help wondering if he's losing his voice. The jacket has a warning: "The horse latitudes designate [ie are] an area north and south of the equator in which ships tend to be becalmed, in which stasis if not stagnation is the order of the day, and where sailors traditionally threw horses overboard to conserve food and water."

This fails to distinguish the Calms of Capricorn from the Calms of Cancer . . . 3,000 miles apart . . . and fails to justify its rhetorical distinction between "stasis" and "stagnation". Repeatedly, the book's erudition is for show and wordplay stands in for meaning instead of standing up for it. After 48 trips to the dictionary I was wearying, until I realised that I should keep by me Volume XV (Ser-Soosy), which yielded "shivs", "sillyhow", "skelf" and "skite".

The poems yaw and loop around. Eleven are back where they started by the end, reprising their openings. The sequence 'The Old Country' comes full circle this way, each of its sonnets along the way having begun by repeating the last line of the previous one. These doubletakes enable Muldoon sometimes both to look back affectionately at his native Ireland and to look askance at the orthodoxy that insisted "Every cap was a cap in hand. /Every coat a trailed coat.

/ Every band was a gallant band."

Louis MacNeice would have enjoyed this witty device while knowing when to stop. Unluckily, Muldoon spins it out to 13 sonnets.

He binds himself in the chains of the sestina, which demands that the words that end the six lines of the first verse be recycled as terminal words in five more verses, then reprised in pairs in a final three lines. The art is to entertain by variation, and he triumphs when "grease monkey" returns as "les ans manques". But when he writes of "breathing indistinguishable now from the sound of a saw / through the breast of a monkey", extravagance becomes nonsense.

His poems are thick with echoes of themselves, of one another, of his other books and other people's books. So the sequence of 90 haikus in Hay (1998) is matched by the 90 here, now called "instant messages" . . . but to what end? The culminating 57 lines of his last poem are hectic with cleverness, and yet there is no explanation why Saint Ignatius, mesotheliomata, a Princeton Reverb, Childe Roland and Cathartes aura are jostling in a single sentence. What is it trying to mean? There is much more pleasure in the simple poetry of seeing how Bubble Wrap is "like bladder wrack" (wordplay standing up for meaning).

But Muldoon always packs the facts odd ways up. His pleasure in the sordid gaudiness of starlings, for instance, has to be reassembled from observations of spotwelds, disco-balls and "boys giving their all / to the sidewalk". A more innocent bird is the subject of "Eggs", which begins: "I was unpacking a dozen eggs / into the fridge when I noticed a hairline crack / at which I pecked". Bafflingly inside-out, this is a fragile miracle of dual perspective.

Self-containment has pleased poets at least since Marvell's Drop of Dew shone "like its own tear".

But Muldoon's reflexiveness is a reflex: "its own shadow", "papiermache / maquette of itself", "a deer dumped in her own numbles", an oyster "in the petri dish / of itself", "watched an old white horse cross the picket / of himself". Horse lassitudes?

A good poet in the doldrums, then. Yet still capable of this:

Plum-blossom blanches at the thought of hard-gotten plum avalanches.