What's new about the Sans Souci is that the food is Italian. What's not new is that the food sounds wonderful but achieves excellence only erratically. It would be safe to guess that nobody ever went to the old Sans Souci for the food. You can do better at the new Sans, but not by much.

On the other hand, the old Sans Souci looked good and so does the new. The green leather banquettes and the gilded mirrors still work a kind of magic that hard-edged modernity does not. Just as people are said to look best in candlelight, pheasant tastes best under chandeliers.

The most obvious change at the Sans Souci (once you have adjusted to the new accent, the pasta instead of bisque d'homard) is the service. At the old powerhouse of a political hangout the ma.itre d'h.otel treated you according to your rank. At the new Sans, everyone is treated like a presidential aide. Paul is gone, but George is there, and he turns the daily specials into poetry and your choice of a menu into something just slightly less important than a cabinet meeting. His is a witty and charming show. (Then there was the waiter whose only wine recommendation was a $40 bottle, and who insisted our choice, a '75, was exactly the same as the '74 he brought.)

The wine list is expensive. It is an excellent wine list, with a broad choice of outstanding Italian wines, including many prominent names such as Gaja; thus, in some--but not all-- cases the high prices are warranted.

The menu is also expensive, with pasta as appetizers averaging $7 and veal or fish dishes generally $13 or $14. On the standing menu, those are the fullest categories; there are also a few beef dishes and a rack of lamb for two, but no poultry unless the kitchen has squab or pheasant that day. At lunch main dishes drop generally under $10, and pastas average $4 as appetizers, $8 as main courses. The most interesting dishes are not on the menu, but rolling off the waiter's tongue as daily specials. Risotto al tartufato fresco. Tagliatelle verde al capriolo. On a single day there might be tortellini with prosciutto, fettucine with smoked salmon, pheasant with almond or walnut pur,ee, squab with muscat grapes, trout saut,eed with sage or stuffed and served in a red wine sauce, scallops with caviar, sweetbreads with Italian mushrooms.

Execution of those wonderful-sounding dishes is, however, unpredictable--except that the pastas are the most likely to taste as delectable as you hope. Fortunately, they are also what sound most tempting as appetizers, since the couple of antipasti we tried--rabbit p.at,e and seafood salad--were dull stuff. But listen to the pastas: Very thin green noodles in a smooth and intense yet mellow brown venison sauce with chunks of venison, a world-class pasta dish. Tortellini of very thin dough with a slightly rough texture that identifies them as homemade, plump with a light and delicate green stuffing, and moistened with just a little cream sauce. Tagliatelle al dente and supple, was tossed with slices of fresh artichoke, in an earthy sauce tasting of mushrooms.

The risottos are a story in themselves. Risotto with fresh truffles is the most elegant sort of simplicity. The fat, chewy grains are cooked to creaminess yet each is distinct, and on top of this buttery mass are shaved fresh white truffles that perfume the table. That was a special; on the everyday menu is risotto with champagne and caviar. A disaster. It's cooked with sweet Asti Spumanti and thus tastes like dessert salted and stained with inky black lumpfish caviar. When we complained, a waiter simply said that was the way it was supposed to be.

As for main dishes, remember that you were warned to concentrate on pastas. Veal was sometimes moist and tender and sometimes stiffly chewy, and likely to be overpounded. The sauce one time was a nondescript brown gravy, another time a thin, vapid tomato pur,ee. Fish varied between rubbery and tough yet fresh of flavor, and tender but fishy-tasting; the trout in red wine had its parts competing with rather than complementing each other. Quail with polenta sounds earthily delicious but tasted limp and gutless. Among all the main dishes, the best by default was filet of beef in gorgonzola sauce. The beef was rare, the sauce delicate and all rather agreeable. But the highlight of the main course was the vegetables; each plate was decorated with a pretty little arrangement of fresh vegetables: a few cauliflower sprigs, a bundle of thin green beans, slices of baby eggplant alternating with carrot slices, halved and perfectly cooked brussels sprouts. Next time I would follow my pasta with a vegetable platter.

The desserts, arrayed on a cart at the entrance, look glorious. Some were good. They were dismal, a chewy and rubbery cake layer under a deliciously intense and fudgy chocolate frosting, for one, a mascarpone cream custard that was excessively sweet and otherwise tasteless, for another. We did have a lovely cassata of fruited ice cream layered with sponge cake on a bed of raspberry pur,ee, and a semifreddo--frozen zabaglione --studded with fruits and nuts that was delicious except that the raspberry sauce overwhelmed it.

The espresso could use improvement, a flaw that is one of a number of unpleasant reminders of the old Sans Souci.

--Phyllis C. Richman TURNING TABLES

Place Your Bets--Gambling pays, as Orhan Soysal, owner of the Full House restaurant at 1317 Connecticut Ave. would no doubt attest. Soysal, former manager of the defunct Da Vinci, used money he won at a poker game to pay for part of the restaurant. Expectedly, his winning hand was a full house, the three aces and two kings that have become the restaurant's logo. In keeping with the theme, the restaurant is decorated in a gambling motif, and little games can win patrons free lunches and drinks. Soysal said he's continued playing poker; his winnings helped with the down payment, a sum that he said "could have been better."

That's Garden With a G--While Rockville's Szechuan Garden has built a steady following over the years, it is hardly news. The new downtown Sichuan Garden, with its imported- from-China chefs, however, has caused a flurry of television and newspaper coverage. We would expect Sichuan Garden to be suddenly swamped, but the Szechuan Garden, too, has reported a sudden improvement in business, a 20 percent increase on a Saturday night at last check. Next we look for the telephone tangle when the Sichuan Pavillon opens.