The Pontiac Bonneville SSE has been on our best-buy list since it first appeared for the 1988 model year.

Too bad the ``don`t mess with success`` formula was violated by Pontiac in the SSE for1990.

Here`s a two-in-one vehicle, a sedan that performs like a coupe. The SSE not only acts sporty, but it also looks the part with deck lid spoiler, front air dam, rocker skirts and wide bodyside moldings color coordinated with the exterior body color. As a final touch, there`s gold honeycomb wheels and gold grille work.

There`s just the right amount of cosmetics so Bonneville doesn`t come across as the family sedan simply because it has four doors. None of the ornamental work makes the SSE go faster, ride more smoothly or handle better, but it makes it look as if it does.

SSE is a four-door, pack-the-family-into-the-rear-seat sedan when it needs to be a hauler, yet a fun-to-drive machine with the spunk of a sports coupe when you feel that need, too.

The 3.8-liter, 165-h.p., fuel-injected V-6 is a steady performer. The V-6 responds quickly to pedal input. There`s a hint of exhaust rumble to provide sound effects that wouldn`t be needed with a 180 h.p. engine. The EPA rating is a respectable 18 m.p.g. city/27 m.p.g. highway.

Soon, however, we would suspect that Pontiac might pack a few more horses under the SSE`s hood to further enhance its performance-car image.

The SSE is earmarked for traction control in the 1992 model year. GM now offers traction control only in the 1990 Cadillac Allante. With traction control, sensors similar to those that tell the wheels not to lock or slip when you apply the antilock brakes tell those wheels to hold firm and not slip or slide on wet or loose surfaces when you accelerate.

SSE comes with antilock brakes as standard, which simplifies the addition of a few more sensors to provide traction control.

For 1990 Pontiac revised the SSE when it should have left well enough alone. It beefed up the front-end structure and retuned the suspension for more isolation from road noise and greater balance in ride and handling. The intent was to feel less ride and handling motion. Honorable goal.

We liked it better the old way, however. It seems as if by reducing road noise Pontiac increased road feel to the point of added harshness. Suddenly you can count each of those tar marks you pass over. The suspension comes across as too stiff. It appears Pontiac put a bit too much focus on SSE`s alter ego coupe.

There`s no problem with lateral motion and almost no lean or sway in corners or turns, but there wasn`t cause for concern before the front-end structure was beefed up and the suspension system stiffened.

In addition to the 3.8 V-6 and antilock brakes, standard equipment in the SSE includes 4-speed automatic transmission, power brakes, electronic level control and variable ratio power steering, headlamp was hers, intermittent wipers, fog lamps, dualpower sport mirrors, 16-inch Goodyear Eagle GT+4 tires, air conditioning, power windows with an express down feature, AM-FM stereo with cassette and clock, remote keyless entry, leather-wrapped steering wheel with radio and heater controls in the hub, rear-window defogger, power door locks, 12-way power seats, tilt wheel and cruise control.

Some of those standard goodies are more annoying than useful. GM`s newest toy, for example, is the express power window. Push the button when you approach the toll booth and the window goes all the way down. That`s fine for toll booths, but a pain at most other times when you only want to crack the window.

We usually enjoy remote keyless entry, the system in which you push a button on the key fob and the doors and/or truck lid unlock as you approach with arms full of packages. The fob on this car had a hair trigger. Several times as we started to walk away, the system would acti ate and the trunk lid would pop open as we pushed the fob into a pocket.

The 12-way power seat buttons are at elbow level in the center console. Too many adjustments and too many buttons. Also, when the wife`s mini-Maltese bounded from her lap and landed on the power seat buttons, dad quickly found himself becoming a steering wheel sandwich as seat motored forward. A power ``off`` button would be useful.

The radio and heater controlsi n the steering wheel hub allow you to adjust the radio dial or temperature setting by touching a button without having to take your eyes off the road. It`s a good system, but there are three drawbacks. There`s no ``on/off`` button; those desiring a driver`s side air bag can`t get one because the radio/heater controls are in the steering wheel hub where the air bag would be; and as a backup all the regular radio and heater controls are in the instrument panel, which makes the panel a bit congested.

Other added touches in the SSE include two power plug outlets at the base of the console for radar detector and car phone; dual cupholders in the center armrest; and an instrument panel button to push for the outside temperature.

SSE base price is $23,994. Our test car added leather seats for $779 and a theft deterrent system for $150 to bring the sticker to $24,923, plus $525 for freight.

>> 1990 Pontiac Bonneville SSE
Wheelbase: 110.8 inches. Length: 198.7 inches. Engine: 3.8 liter, 165 h.p. V-8. Transmission: 4-speed automatic. Fuel economy: 18 m.p.g Base price: $23,994. Strong point: Dual personality, sporty styling. Weak point: Revised suspension, front end to stiff. >>