NOVI, Mich. -- General Motors officials say they are poised for a good year with their highly acclaimed new Cadillac STS and Chevrolet Corvette models, and there are high hopes for the all-new compact Chevy Cobalt as well.

But company officials who attended the firm's recent 2005 full-line preview at its test track in suburban Detroit are the first to admit that not all of its divisions share in the wealth of new, up-to-date products. They say they still have their work cut out with the Buick and Saturn divisions in particular.

Indeed, the nation's largest automaker said yesterday that sales of Saturn's Ion compact car were so soft that it would cut 300 to 400 jobs and slow production at its Spring Hill, Tenn., assembly plant next month, with the number of work crews falling from three to two. The plant now operates with three crews of 300 to 400 workers over two shifts, six days a week.

Like all other car companies, GM is facing intense competition from foreign firms that have speeded up the new model introduction schedule. As a result, pressure is great to replace or renew its models much more quickly than in the past. A cycle of two to three years is about all that companies dare endure if they don't want to be left in the dust.

GM's difficulties are reflected in the Detroit-based company's market share and sales figures. Its share of the domestic market through July had dropped to 27.4 percent from 27.9 percent a year ago, and sales for July rose only a scant 0.1 percent, vs. a much stronger month in 2003 -- and that came even as sales this year were often fueled by rebates of up to $6,000 on some models.

Standard & Poor's this week warned that it may cut GM's debt ratings to near junk levels over the next two years if profits in its mainstay auto business don't improve. S&P analyst Scott Sprinzen said the ratings agency was concerned that most of GM's profits were being driven by its financing and auto loan business instead of car sales, and said the second half of the year was "not a pretty picture with respect to GM and the auto business."

GM officials hope to counter the negative trends with improved productivity at its domestic plants and with increased sales of new models that are earning rave reviews. The STS, for example, already is considered by some car writers to be the first truly viable competition from this country for the BMW 5 Series and similar models.

Judging from a test drive, the STS will be a formidable opponent to the best Europe has to offer in the luxury car market, where profit margins are much higher.

It handles aggressively and accelerates smartly with a growl appropriate for cars in this category. Inside, it has an elegant, richly appointed, but subdued interior that will remind one of a Mercedes E-Class. The usual bells and whistles found on other luxury models are in evidence on the STS.

In the new Corvette, which replaces the C-5 generation that Corvette enthusiasts generally consider to be the best of the brand, the improvements are noticeable. It has a much quieter interior absent of squeaks and rattles, better construction and a richer interior. It's faster too, with a 400- horsepower V-8 engine.

And the Cobalt takes Chevy's compact entries a notch or two up largely because of the quality of the interior and workmanship, which are greatly improved over the Cavalier. A high-performance model of the Cobalt is likely to win over a number of "tuners," young adults who personalize and soup up compact cars.

But GM still lags in several divisions. Buick has popular models in the Rendezvous, the new Rainier SUV and its new LaCrosse sedan, which replaces the Century and Regal. But its bread-and-butter LeSabre and Park Avenue sedans have grown long in the tooth, and GM knows it.

The automaker is spending $3.2 billion to create an all-new product portfolio by 2006, and replacements should hit the market in a year or two. It's thought that Buick also will put into production the popular Velite convertible concept car -- a classy, elegant model that drew raves at this year's New York International Automobile Show.

The Pontiac division is better off, with the new G6 sedan to replace the Grand Am. And as sleek as the sedan version looks, pictures of the coupe and convertible models posted on Web sites suggest they may be big sellers for the division. They look a lot like the popular Infiniti G35 Coupe and the Audi A4 Cabriolet. Pontiac also is excited about a hot new sportster, Solstice, which will debut in the fall of 2005.

Of course, it's not guaranteed that new models will necessarily be better models. GM already is catching flak for its Chevy Uplander, Buick Terraza, Pontiac Montana SV6 and Saturn Relay -- minivans that don't look any different from the models they replaced.

It's particularly unfortunate for Saturn, which could use a hit. Unlike Pontiac with its Solstice, Buick with its Velite concept car, Cadillac with the XLR convertible and Chevy with the Corvette, Saturn lacks a knock 'em dead, got-to-have-it car to bring buyers into the showroom.

And although its Ion compact sedan is greatly improved over previous models, it still makes no new inroads in design, styling and content. It doesn't help that Saturn also lacks a model in the highly popular intermediate sedan segment of the market.

While it waits out the storm, GM hopes to improve things in the next year or two. It's planning to double Saturn's offerings within the next 18 to 20 months, including an all-new roadster that will bow in 2006.

But perhaps the biggest challenge for GM is simply getting the public to believe that it has changed, and that quality control and reliability for its products have improved. Other American automakers say the same thing as they try to come back from disastrous reputations built during the 1970s and 1980s.

Lucky for GM, it has finally realized the importance of strong styling differentiation within its lineup and is providing the public with edgier, eye-catching styling. It also has better quality going for it -- the Buick and Cadillac brands were among the top five nameplates in the latest J.D. Power & Associates study.