Hyundai Repair: Hyundai terracan rough idle, torque converter clutch, throttle position sensor

Question
Hi, I have a v6 2002 Terracan (petrol) Australia.   Upon starting the car the rpm dips then rises again. When it idles it idles very low and shudders, almost as if it would stall.(worse with air con running).Almost sounds like an old diesel. Sometimes when the car is at higher speeds there seems to be a sudden lack of power and then it corrects itself. The dealer found an ignition coil code in 2007, but began to run rough again. They have replaced one coil pack and after that didn't fix it replaced the remaining 2, as well as leads.  A lot of money spent and problem remains. They recently looked at it again, re adjusted a few things but still same and now when I drive between 20-40 km the rpm needle rises and drops erratically, and the car surges in power??! so we now have this problem as well as the existing idle problem. any ideas?  The dealers seem to be at a loss

Answer
I'm going to hazard a guess that you have a problem with the throttle position sensor.  This is the primary sensor the engine control module (ECM) uses to determine whether to idle the engine.  In addition, the transmission control module (TCM) uses input from this sensor to determine when to shift and when to apply and release the torque converter clutch.  All the symptoms you list are consistent with a defective throttle position sensor.