Open Monday through Friday, 11:45 a.m. to 2:45 p.m. and 6 to 10:30 p.m.; Saturday and Sunday, 5:30 to 10:30 p.m. AE, CB, MC, V. Reservations. Prices: Pastas $6 to $7.50, main courses $8 to $11.25 at dinner.

Il Nido is full of contradictions, the most mysterious being that a restaurant can make so many mistakes and still leave a diner feeling elated for having found such a good restaurant. I can't explain it, I can only relate it.

First, it is a treat to encounter such visual delight in the middle of an upper Wisconsin Avenue shopping strip. The restaurant is small, lined with banquettes of striped coarse cotton against a wall of glossy burgundy tile. The paintings on the other, pale walls, are art works of character. The room is crisp and modern, sophisticated as few other new restaurants are. During the day, the sun streams in through a picture window etched with a nest of baby birds, indicating to the astute that Il Nido means nest in Italian.

The second treat is the menu, its 20 main courses including not only the routine - piccatina al limone, frittura mista, saltimbocca alla Romana - but the unexpected - pork roasted with milk and spices, lamb cooked with eggs, fish Livorno style. The pastas, all eight of them, include not a single overexposed standard. There is no fettucine Alfredo, no tortellinialla panna, no manicotti. Instead, one finds maccheroni alla chitarra and risotto di mare, rigatoni Calabrian style and lasagna Teramano style.

Along with the consideration of the menu, however, come the first of the disappointments. No half-portions of pasta, which means that you must share a whole portion (and have to choose only one of the eight impossible choices) or order pasta as a main course (an alien concept in a true Italian restaurant) or order too much food (at too-high prices to make that possibility at all attractive). Second, the maitre d'hotel, though a concerted effort can slow him down and make him pay attention, tends to railroad you through a quick order of what he thinks you should have. One day that was grilled striped bass, which turned out on the plate to be red snapper. It wasn't bad, but left us mystified that somebody would push the striped bass when they didn't even have it. As long as we are going on about the service, it must be noted that at one table the appetizer and main course were brought together. The service has a manic quality, swinging from festive and attentive to brusque. The headwaiter might patiently describe the specials and thoughfully mention their price, might grate cheese over your pasta after deftly portioning it, then might forget your silver. The meal is always saved, however, by the conscientious young busboy who is not only constanly attentive, but lovingly describes the desserts that the headwaiter has only mumbled about in passing.

The food, too, has certain mood swings, though usually it is up. One day the fried zucchini deserved zucchini heaven; it was julienned, lightly battered, so crisp and fragile it tasted almost like a mirage. Another day the zucchini was sliced, limp and greasy.

But the food can be wonderful. So, having had several lessons in assertiveness from the headwaiter, I will try to railroad you into ordering the best dishes. Appetizers are not the restaurant's strength. Seafood antipasto is a melange of briny freshness, but lacks zip. Fried mozzarella and soups are good, but the kitchen has more exciting food to offer.

Delve deeply into the pasta section. Risotto, that creamy mass of slowly cooked fat rice, is studded with seafood and permeated with subtle flavor and painstakingly achieved texture. Lasagna Teramano style is the lightest layering of crepes, tomato sauce and cheese, pure silk in this world of clumsy polyester lasagnas. The house specialty, fregnacce, is also a crepe dish, this time wrapped around a cloud of ground veal and moistened with tomato sauce. As for the wonderful wire-cut maccheroni alla chitarra, the delicacy of the noodles could use a more vibrant tomato sauce. Rigatoni is happily paired with eggplant and tomato, but can't compete with the more intricate, delicate pastas.

If you only get to Il Nido once, take a generous friend so you can taste both the frittura mista - its shrimp and squid lightly floured and cooked just so that they crackle and burst in your mouth - and the veal scallops - beautifully cooked impeccable meat, however it may be sauced. If there is a roast chicken, you will have a chance to reacquaint yourself with the real thing - crisp and oozing meat juices - ane day accompanied by sausage-stuffed zucchini. There are other, even more unusual dishes worth seeking: shrimp and squid are grilled on skewers for a charcoal-scented variation on the frittura mista theme, and served on a romaine leaf so that they look like a Japanese still life. Agnello cacio e ovo is chunks of lamb buried in a broth-flavored, custardy egg mixture, a homey, comforting dish. Roast veal can be excellent, though sometimes slightly overdone. The roast lamb, while boldly seasoned with garlic and rosemary, suffers from the Italian habit of overcooking.

Sufficient attention is paid to accompaniments to make them real assets. Pan-roasted potatoes and braised greens one day, sauteed mushrooms another day. Salad is a simple matter of fresh cold greens with respectable oil and wine vinegar, well tossed and served on a plate. The bread needs improvement, but the coffee does Italy proud.

Il Nido's wine list is expensive, each being just a dollar or two below the outrages of the downtown Italian restaurants. Though the list is small, it is broad, with the Abruzzi wines - when they are in stock - being particularly interesting and, at about $9.50, not too expensive.

Dessert is an elusive course. Never did I find the highly touted cannole available. Zuppa inglese turned out to be a pretty good strawberry shortcake lightly laced with rum. Otherwise, each visit showed fairly good but hardly Italian cakes. But there was, thank goodness, always the semifreddo, a very creamy orange-flavored frozen cream with a Grand Marnier sauce and a tangle of candied orange peel. A sumptuous finale.

The prices reflect the restaurant's ambitions. With main courses averaging over $8 and pastas averaging $7, it is easy to run up a bill of over $25 a person. Mozzarella in carrozza costs $3.75, the semifreddo $3. Thus, through its prices Il Nido sets itself as a challenge to Washington's top Italian restaurants. But its kitchen work backs up its challenge. Il Nido's flaws are superficial sloppiness here and there; it has the basic talent to compete at the top.