The torque converter is unlocking intermittently. The clue is your observation of the 200 rpm change which is exactly how much engine speed will change. The torque converter unlocks under heavy acceleration to create a doubling of torque, and it will unlock in what it thinks is a preparation for coming to a stop. Releasing the accelerator pedal to less than about 10 percent will initiate an unlock. Tapping the brake pedal will do that too, although that will also cancel the cruise control. You can duplicate the symptom by holding the accelerator perfectly steady while driving in third or fourth gear over about 45 miles per hour, then briefly tapping the brake pedal. You'll see engine speed go up 200 rpm, then drop back down one or two seconds later when the torque converter relocks.
When this problem starts occurring on its own, use your toes to hold up the brake pedal. If the problem stops, check the adjustment of the brake light switch.
In rare instances it is possible for the throttle position sensor to cause this but usually it's because the signal voltage pops to 0.0 or 5.0 volts. Both of those will be detected as unacceptable conditions, set a diagnostic fault code, and turn on the Check Engine light. If that's not happening, most likely the throttle position sensor isn't the cause.
You can also connect a scanner to view live data to see what is triggering the unlock. The status of the brake light switch will be shown along with various sensor data. By recording a few seconds of information when the unlock occurs, you can play it back slowly, frame-by-frame and see the data in more detail to see what's changing.
Tuesday, February 4th, 2014 AT 3:02 PM