99 intrigue trans problem?

My fiancee has a 99 3.8 intrigue, purchased new, w/130,000 miles on it,
overall the car has been pretty good and dependable. A few weeks ago, the car started shuttering when driving down the road, sometimes it would do it from the point of takeoff until you reached 50-70 mph, sometimes it would start as you were driving down the interstate. I assumed the transmission was slipping, but after driving the car, the rpm hand is not fluctuating, as this is going on. She had never changed the fluid, so we had that done, and it drove fine for a week or so then intermittently back to the same thing. The mechanic said possibly he could put some kind of anti-slip stuff in there and it might help, but no guarantees. I am just wondering what the problem is. Is it the trans, torque converter? Help is appreciated.

this is a known issue with the 4t65E tranny in your car, as well as many other GM models (grand prix, venture/montana/sillouette, regal, etc). This sounds like the valve body needs to be replaced. Is the car shifting hard (1-2 shift or garage shifts)? If so, the PCS (transmission Pressure control solenoid) needs to be replaced. The valve body repair will set you back $600 or so, and the PCS is 300-400 bucks to replace. Doubtful you will need both, but one or the other is required.