2001 ford taurus running rough at low rpm

car at low rpm or at cruising speed has a stutter or flutter feel as if trying to shift. it has been on diagnostic machine and no problems are found. ford shop put in new plugs and wires but problem persists and even seems to be getting worse. problem is very subtle and car is driven daily with no problems, however i am concerned about transmission or torque converter causing these rough spots and will car remain dependable? thank you for your time Doug Privitt