My 03 V6 does the same thing and I tried to get the dealer to diagnose it a year ago, even to the extent of calling in the Field Service Tech.

They hooked up all the electronic gizmos etc., and found nothing wrong. As I sat in the vehicle and let him know every time I felt the "kick-shudder" , about 2-3 seconds apart (in park or neutral), he said nothing is registering on the device. Consequently, it doesn't point them in a particular direction to pinpoint a cause. At least that's the official answer.

You will probably notice the tach does not move when it happens. I think it's an internal imbalance that happens to cycle and repeat at idle (in park or neutral). When put in gear, it stops. Rev the motor even slightly, it stops. Turn on the A/C, it stops.

Idle just happens to be the "sweet spot" of the reasonating-cyclical-imbalance.

This is my theory anyway. If you find a cure, please post. Lots of V engines out there suffer from this annoying malady.