SOMETIMES, productions try too hard: they don't interpret, they invade. And when a production of Mozart's Cosi fan Tutte features someone in a rabbit costume with a 35ft penis, the invasion is most likely terminal - as it is in Scottish Opera's zoomorphic new Cosi at the Theatre Royal, Glasgow.

On a practical point, I should say that the 35ft penis is worn looped over the shoulder like a hosepipe. Mercifully, the rabbit never gets excited: if it did, there'd be a breach of health-and-safety regulations. But what is this rabbit and its flaccid tackle doing on the stage at all? Why is there some kind of low-energy orgy happening around it? And why does Dorabella then sing "E amore un ladroncello" sitting fully clothed in a tub of water?

The director, Stewart Laing, would probably explain it all as an attempt to create a world of counterfeit, teasing perversity and role-play: fair enough for a piece notoriously hard to take on Mozart's terms and easier to reinvent as the opera Iris Murdoch never quite got round to writing. Cosi's core idea of love tested by deceit, provoked into betrayal, and then pacified in a peremptory paean of off-the-peg forgiveness sticks in late- 20th-century gullets. Almost every modern production massages it to some degree beyond the composer's expectations, into a contemporary statement on the way relationships develop.

Jonathan Miller's Armani-clad contemporisation for the Royal Opera was subtle; ENO's Fifties New Look staging was briskly forthright. But this job is coarse and muddled: an attempt at post-modern chic that misfires. Its semi-abstract set looks like somewhere backstage at the South Bank Centre, only twice as shabby. Its definition of character is vague and inconsistent. And while the staging suffers from too much licence, the music suffers from blind faithfulness. Most Cosis trim the recits in the interests of dramatic shape. But here, the conductor Nicholas McGegan keeps just about every note.

Consolation of sorts comes in the voices. There's a baritone of real distinction and dimension in Peter Mattei, whose Guglielmo almost saves the show. There's also an assertive Despina from Lisa Milne, and some attractive singing from Claire Rutter (Fiordiligi) and Iain Paton (Ferrando). That none of them seem particularly involved in the theatre of what they do is understandable. When you're settling into the Act II Scene 2 ensemble and a rabbit with a giant willy walks past, how do you respond?

There's a touch of indifference among the performances in Handel's Xerxes at ENO; but here it's studied, purposeful, and part of the show which somehow stays perpetually fit and fresh, as wonderful a curiosity as when it first played 13 years ago. Nicholas Hytner's production is an example of post-modern chic that works: a fantasy quotation of an 18th- century world into whose ordered, quietly obsessive calm the passions of the central characters explode like gift-wrapped Semtex. Tightly whimsical, fastidiously designed, it finds the tone juste for this kind of semi-serious baroque conceit. And with the likes of Yvonne Kenny and Ann Murray in its casts, it has assumed a sort of classic status. One of ENO's flagship shows.

The cast this time is good too: mostly new, and with Janis Kelly claiming the stage as a consummately eloquent Romilda. Sarah Connolly, by contrast, doesn't quite stand her ground in the title role; but it's a lovely voice, more charming than Ann Murray's, if less expressive. I'd say the same of Artur Stefanowicz's Arsamenes: his predecessor, Christopher Robson, was more exotic, but less lyrical. And Susan Gritton is pure joy as Atalanta: bright, keen, touching and enchanting. If Noel Davies in the pit had shown more of those qualities, it would have been a perfect evening.

But I still left happy which is more than can be said of the first- night audience at the new Tristan und Isolde, which opened last week at the Bastille in Paris. I've rarely heard booing so fierce or disciplined: it hit the stage like an accelerating bus. The chief target was Stein Winge, the Norwegian director responsible for last year's ENO Flying Dutchman. If you recall that show, you'll know that Winge's style is upfront impact, generated by some stunning piece of apparatus on the stage that isn't quite so stunning two acts later when it's still there. His Paris Tristan is a variation on that theme, dominated by a vast skeletal structure that serves nicely as the hulk of the boat in Act I, but not so nicely as a sort of bower for the lovers in Act II. At least it disappears in Act III; and although the accompanying production ideas aren't remarkable, they don't merit the reaction they got.

I can only think the audience was transferring to Winge its frustration at not getting the cast we had all come for, notably Anne Evans, one of Britain's proudest exports to the Wagner world. It was supposed to be her Paris debut. She was indisposed. So we had Carol Yahr instead, the American mezzo-turned-soprano known in this country for Wagner roles at Scottish Opera; and it wasn't much of a substitute. Her Tristan, Wolfgang Schmidt, wasn't so pleasing either. Which left the laurels to Rene Pape (a crisp, dark King Mark) and Jane Henschel (a resonant Brangane, who rose above the costume designer's vision of her as a garden gnome). James Conlon, the American music director, conducted with luminous distinction; and the orchestral sound was healthier than anything I've heard from a French orchestra in a long time.

The sound of the New World Symphony Orchestra, on tour to the Barbican this week, was more equivocal. I wrote about this American training orchestra a year ago, after a visit to its home base in Miami, and thought then that it was superb: a precedent for what our own Young Musicians Symphony Orchestra might be, if it only had the resources, dates and connections that someone like Michael Tilson Thomas (NWSO's founder-conductor) brings in his gift.

Tuesday's concert, by comparison, had weaknesses. The ensemble playing wasn't as dynamic as I remembered, or the solo playing so virtuosic. But that said, it still outclassed anything Britain could offer at the same intermediate level. There was obvious musicianship, a beautifully tailored programme (Barber, Ives, Debussy, Adams), and the electricity that Tilson Thomas always generates.

'Cosi': Glasgow Theatre Royal (0141 332 9000), Tue, Thur. 'Xerxes': Coliseum, WC2 (0171 632 8300), Fri & to 6 Mar.