Solomon R. Guggenheim (1861-1949) was born in Philadelphia of a Swiss-Jewish immigrant family who had made shrewd investments in the mining industry. He and his wife Irene had been collecting old masters when they met Hilla Rebay, an Alsatian baroness who was a fervent theosophist and believed in art as a spiritual experience. It was best expressed in abstract art, she taught the Guggenheims. Under her tutelage they began to collect unknown but wildly talented people, and became entranced by Wassily Kandinsky, whose pictures they bought in droves.

They founded the Museum of Non Objective Art at 24 East 54th Street, New York, in 1939. Once a car salon, it was hung with vast silvery paintings of floating globes by Rebays teacher, Rudolf Steiner, and piped through with music by Bach. In the 1940s I was allowed in by the guard, another student, Bob De Niro. We enjoyed the museums air conditioning, then a rare thing in a hot, humid New York summer, almost more than the paintings. Eventually the Guggenheim couple and Rebay approached the most daring architect of the moment, Frank Lloyd Wright, to build a proper museum to house their acquisitions. The eccentric edifice, winding skyward like a giant apple peel, completed only in 1959 on the corner of 89th Street and the park, is now a New York landmark. In the beginning it was ridiculed by a sarcastic press and artists hated it, because they thought the architecture overwhelmed their work.

Very much later, in 1995, a Guggenheim museum of even greater spatial extravagance was built by the architect Frank Gehry to billow like a great airship over the industrial town of Bilbao in Spain. It too raised an outcry as well as much admiration.

Solomon Guggenheim had a niece, Peggy. The little rich girl expressed her first thrills in collecting by founding the Guggenheim Jeune gallery in 1939 in London. But when the war came she took on her hometown, New York, and immediately set it by the ears. Her Art of This Century gallery, designed by an avant-garde architect, Frederick Kiesler, opened in 1942 at 30 West 57th Street, in what had been tailors lofts.

It was a fantasy-land for fantastic surrealism, with walls out of kilter, paintings attached to ropes, a spooky corridor like a sinister penny arcade, see-sawing chairs and a soundtrack of a speeding train. It first dazed, then enmeshed you. Peggy filled the gallery with tidal waves of her divergent discoveries of Catholic taste, never forgetting to favour her surrealists. I saw my first Jackson Pollock show here, I saw my first show of only women, I saw Motherwell earnestly lecturing unsuspecting art students.

Here at Peggys at least art was never solemn.

After the war, in 1949, Peggy astonished Venice by descending on it with her many-featured brood of artworks and settling in a palace on the Grand Canal. Her museum there became another modern art pilgrimage station.

After Peggys death in 1979 her collections were added to those of her uncle, which had already been augmented by Justin Thannhouser, Karl Nierendorf, Panza di Biuma and other hoards of perspicacious millionaires.

Today it is not only art-world people who are hip to the great Guggenheim name. It stands for the great churches of modern art, the new religious experience, where you enter impressive modern architecture and whisper in awe before much reproduced and hallowed works.

So when this show was planned in Rome it raised high expectations. But they are not fulfilled. Entering the exhibition, the savvy international crowd may take it as a refresher course, but young people and the innocent local public may take it as a snub. What is all the fuss about? Once more the Scuderie del Quirinale has let us down with a helter-skelter selection chosen by assistant curators.

The itinerary through the 40-odd enticing titbits is a maze, perhaps to be untangled by explanatory earphones. There is an indifferent minor work by one master here, one art direction emphasised there and some confusing confrontations. There is work by only one woman. What is unforgivable is that the greatest achievement of American art, the New York abstract expressionist revolutionaries, are left out there is not a single Willem de Kooning, Franz Kline, Phillip Guston or Jack Tworkow. Nor is there much from the recent decades of conceptual or minimalist art.

However, if you let yourself wander about, and will yourself to be surprised, some of the old icons can shine out at you again with the flash of first recognition. For instance Pierre Bonnards glorious Dining room on the garden is a symphony in golden colour. Piet Mondrians Lozenge of 1930 is a diamond-sharp abstraction of breathtaking simplicity. Constantin Brancusis sweetly honed-down marble Muse is like an egg, gently teetering sideways. Alberto Giacomettis drawing of his brother Diego is ice-splinter clear and spacious, his sculpture called Nose both sinister and hilarious. Lszl Moholy-Nagys hanging Dual form is an evocative tangle of glitter. One of Picassos least ambitious oils, the sleeping blonde, has skin painted in Mars violet squeezed fresh from the tube. There are two quite early Kandinskys, one of a little bright landscape, another a rain-scape. A docile steer by Franz Marc curls up meek and velvety. Georges Seurats country people are elegantly defined by sunlight. Robert Delauneys huge disks are pleasing essays in rainbow colour. Among the Pollocks there is one in snake-spit whorls, there is a whispy Cy Twombly, a slightly inflated but intriguing Fibonacci series conceit by Mario Merz. All the above are my favourites.

There are some who have taken too few risks Georges Braque, Fernand Leger, Joan Mir, Marc Chagall, the inevitable Max Ernst and so on who may soon appear as too overrated, and Giacomo Balla and Giorgio De Chirico have also been thrown in.

Does this show really explain the true gospel of modern art according to the Guggenheims? It does not. But once you let go of looking for order or a coherent historical outlook and balance in this random accumulation, some of the morsels from the great Guggenheim banquet are just marvellous.

Capolavori del Guggenheim. Il grande collezionismo da Renoir a Warhol. Until 5 June. Scuderie del Quirinale. Via XXIV Maggio 16.

Tel. 0639967500.