DTC P1518 = Throttle Actuator Control (TAC) Module Serial Data Circuit
The diagnostic monitors the accuracy of the serial data transmitted between the TAC Module and the PCM. If the PCM/TAC module detects a loss of data or invalid data, this DTC sets.
That's based on the 2000 factory manual. Don't have access to a 2001 manual. Doesn't sound like something related to the coils or ICM. It can be related to low system voltage. Does the dash gauge show a healthy 13+ volts at all times.
And you indicate you "looked at all the connection"...... do you mean the wiring between the PCM and the TAC, and the power supply and ground at the TAC.
You can download a free copy of the 2000 manual from the link in the top "pinned" thread on the topic listing for the Fourth Gen (this) forum. Maybe one of the late model V6 car owners can offer you more help than this. I know several of them have had problems with the electronically controlled throttle system.