In August, the independent testers at Consumer Reports handed all-electric automaker Tesla one heck of a rave review, giving its heavily hyped Model S the best performance rating ever, a staggering 103 out of 100.

Now, the ratings giant has dinged the showpiece from America's youngest car company with a far less glowing endorsement, withdrawing its recommendation due to drivers' complaints of a "worse-than-average overall problem rate."

Shares in billionaire Elon Musk's prized auto upstart plunged nearly 7 percent Tuesday on the news, as investors worried that reliability woes could damage the reputation of a groundbreaking luxury sedan that is priced fully loaded at about $130,000.

But drivers' concerns over the Model S — touching everything from a squeaky sunroof to warping brakes — could pose an even more long-term threat for Tesla, which is ramping up the car's production, unveiling new vehicles and fighting to compete with its bigger, more established rivals.

For its annual car-reliability survey, Consumer Reports said it heard from about 1,400 Model S owners who complained of an "array of detailed and complicated maladies," both in the car's unique touches, like its sprawling touchscreen dashboard, and in its more commonplace machinery, like windshield wipers and wheel alignment.

Though Consumer Reports reiterated that the Model S remained its best-performing car ever tested — its new "Ludicrous Mode" pushes it from 0 to 60 mph in less than 3 seconds — reviewers said the car's reliability seemed to have actually worsened from the previous year, including in its steering and suspension.

"People are saying they love the car," said Jake Fisher, head of auto testing for Consumer Reports, in an interview, "but when we ask about how they’ve been holding up, we're seeing some major issues, in things like the electric motors and charging system."

The problems, reviewers said, were typically fixed at no cost to drivers under Tesla's four- to eight-year warranties. And surveyed Model S owners remained happy, even amid flaws: Almost every respondent said Tesla had been quick with responses and repairs, and 97 percent of those surveyed said they would buy the car again.

"Consumer Reports also found that customers rate Tesla service and loyalty as the best in the world," a Tesla spokesperson said. "Close communication with our customers enables Tesla to receive input, proactively address issues, and quickly fix problems. Over-the-air software updates allow Tesla to diagnose and fix most bugs without the need to come in for service. In instances when hardware needs to be fixed, we strive to make it painless."

The disappointing report card comes less than a week after the Model S' software was updated over-the-air to enable "Autopilot," one of the first ways the driving public can try out many of the touches of a driverless car. When activated, the car can automatically steer through turns, hit the brakes, drive at highway speed and even parallel park.

Inside a Tesla factory

An employee positions a Tesla Model S automobile during battery pack fitting on the final assembly at the Tesla Motors Inc. factory in Tilburg, Netherlands, on Thursday, Oct. 8, 2015. Tesla said it delivered 11,580 vehicles in the third quarter, meeting its target after Chief Executive Officer Elon Musk handed over the first Model X sport utility vehicles just before the end of the period. Photographer: Jasper Juinen/Bloomberg (Jasper Juinen/Bloomberg)

The reliability concerns underscore just how tough it is to build a car, and a car company, from scratch, especially when it comes to the kinds of electric-motor, infotainment and software upgrades on which the automaker has made its name. The only other vehicle now made by Tesla, the recently released Model X SUV, was delayed for years and will sell for a higher price than analysts expected, and Musk has called it "one of the hardest vehicles in the world" to build and produce.

Some market-watchers like Efraim Levy, an analyst with S&P Capital IQ, expected the concerns wouldn't likely damage Tesla's growing fan base. "This report will deter few who otherwise would have purchased a Tesla vehicle," Levy said in a Tuesday report. "Still, the premium priced volatile shares can move widely on chinks in Tesla's image armor.

But the news bodes ill for investors betting that Tesla can turn its niche product into a mass-market winner. Only about 33,000 of the Model S have been delivered to buyers in the first nine months of the year, and high repair costs could both hurt the carmaker's bottom line and pose a distinct threat if it hopes to deliver more cars down the road. Tesla has pledged to bring a $35,000 car to the masses, called the Model 3, within the next three years.

"It’s one thing to have a quirky, problematic car that sells 20,000 units per year to wealthy people who probably own at least one backup vehicle," a Consumer Reports reviewer wrote. "It’s quite another when Tesla scales up to its 2020 projection of 200,000 U.S. Model 3 buyers, who may not have the luxury of being so forgiving."