Time is running out for a very special Jaguar. Although the XJS grand touring coupe and convertible will be back for the 1995 model year, neither car will be available with a five-speed manual transmission. That's how this week's test car came equipped. And let me tell you something: If you yearn for the days when Jaguar built real sports cars - cats that would leap from 0 to 60 mph in 6.8 seconds - you might want to hustle down to your local Jaguar dealer and see what the XJS with its German built five-speed can do. Like me, you might be shocked. The XJS always has been an exceptionally smooth and somewhat-quick grand touring machine. But a souped-up Camaro or Mustang could easily leave it at a stoplight. With the five-speed, you can wind up the Jag's race-proven engine and unleash the big cat's full performance potential. Not many cars will be able to out-accelerate the XJS five-speed. PERFORMANCE Jaguar claims that the 1994 models are ''the best Jaguars ever built.'' This is no empty sales pitch. For each of the past three years, Jaguar has made dramatic gains in quality. True, Jaguar had a lot of ground to make up, but bankrolled by parent company Ford Motor Co., Jaguar now has the resources to build-high quality automobiles. And based on this week's test car, that's exactly what the British automaker is doing. The 24-valve straight-six engine in the XJS convertible idles so smoothly that you would swear it isn't running at all. Lexus has nothing on Jaguar in this area. Of all the Jaguars I've driven over the years, this week's test car felt the most solid - and trustworthy. I drove the car hard nearly every time I got behind the wheel. The 4.0-liter, 219-horsepower aluminum engine can take one heck of beating. Many times I wound up the silky-smooth engine close to the red line on the tachometer. Never once did the car miss a beat, nor did the temperature or oil pressure gauges come anywhere close to veering into their danger zones. You can't say that about older Jags, which were known to overheat, stall and suffer catastrophic failures for no apparent reason. Those days are gone. What makes this week's XJS special is its manual transmission. Jaguar officials thought a stick shift would appeal to ''enthusiast'' drivers. And although the German-built five-speed Getrag gearbox does transform the car, many buyers apparently feel that they shouldn't have to manually shift a $60,000 automobile. Jaguar officials say only about 100 customers in the United States have bought an XJS with the stick shift and that the option will be dropped by the end of the 1994 model year. Having driven all versions of the XJS (the six-cylinder also comes with an automatic transmission, and there's a 12-cylinderengine with an automatic), there's no doubt in my mind that this week's test car is the one I would most want to own. Performance-minded drivers will fi nd that it is by far the most fun and enjoyable version to drive. The clutch is firm, tight and fast, but with a positive feel. It doesn't take long to get used to the pedal and learn how to shift quickly. The shifter slips smoothly and easily into each gear. Because the engine pulls so strongly and revs to 6,000 rpm, you find that you don't have to shift as often as you might expect. Despite my heavy foot, fuel mileage was excellent. The test car delivered 18 mpg in the city and 24 on the highway. Not bad for a car that weighs more than 2 tons. HANDLING Our test car came with Jaguar's optional ''sports pack'' suspension system, which includes fat 16-inch Pirelli tires mounted on special wheels, high-performance shocks, a front anti-roll bar and stiffer springs. The XJS feels as if it is connected to the road the same way a roller coaster is connected to its tracks. No matter how hard I tossed the XJS into a curve the body always stayed oised and the tires never once lost their grip. The XJS is a fairly wide car, an d I think that, along with some extraordinary fine-tuning of the suspension system, is the secret to the Jaguar's lithe and agile road manners. The power-assisted steering is a little too light for my liking, but it is quick and, because it takes so little effort to turn the wheel, it makes the car seem that much more responsive. The four-wheel power-assisted disc brakes are the best I can remember on a Jaguar. Touch the pedal and they bite hard, slowing the car quickly and without fuss. FIT AND FINISH I have no complaints with the way the XJS is assembled. Parts are fitted much better by Jaguar than they used to be. However, there are areas where the XJS is really showing its advanced age. The XJS, in production since 1975, has been improved over the years, but the overall design of the interior still feels as if it belongs in the mid-70s, not the mid-90s. Ergonomically, the XJS is something of a mess. You can get reasonably comfortable in the car, but in no way will the XJS coddle you in the same manner as, say, a Lexus SC 400 sport coupe. The tilt steering wheel is too close to the edge of the driver's seat - making getting in and out a tight squeeze. When raised, the wheel obscures the instruments. Because the front and rear seats actually touch, there is absolutely no legroom in the rear for passengers. In all likelihood, Jaguar added the rear seat so that the XJS convertible would be classified as a four-seater, thereby qualifying for lower insurance premiums. Insurance companies tend to charge a premium for two-seat sports cars. The rear area must be viewed only as a place to store small items. The electrically adjustable front seats, covered with tan leather, were quite comfortable, but a bit narrow. Those with a large derriere might find the XJS a tight fit. The center console, though beautifully trimmed in wood, is home to numerous switches and buttons. The electric door locks clunk loudly. And yet from the outside, our British Racing Green test car with its tan cloth convertible top was simply gorgeous. Raising and lowering that top can be done with the flick of a button. The car is quiet, top up or down, and visibility is good. This year, the XJS has both a driver's and passenger's air bag. Despite having to make compromises inside, the XJS - especially with the stick shift -is a very rewarding vehicle to drive. Truett's tip: The XJS convertible, when equipped with a five-speed manual transmission, is fast and mean. And like fine wine, the XJS just keeps getting better with age.