Taurus with a cough. Any assistance?

Ford Taurus SE, 2000, almost 60,000 miles, 3.0 V6. I was sitting at a stop light when I felt the car stutter-it didn't stall, but the tachometer was moving up and down. I figured I'd nurse her home n' check it out. Sitting at another intersection closer to home, it stalled. It seemed to stutter only when I used my turn signals. Not knowing anything about cars, I decided to change the spark plugs (interesting side note, the dealer had one plug of the completely wrong size in). It seemed to help at first, but now it only delays the eminent stall. Then I thought alternator, but when I had it checked out, the tester said it was working fine and the parts store employee said it was probably an O2 sensor. Could anything else be causing this problem?