The rough idle guessing game.
1. Coil or plug as you indicated.
2. Air or vacuum leaks in throttle body.
3. Dirt in fuel injector stopping pintle from closing.
4. Throttle bodies out of balance.
5. Phase seperated ethonal/gas leaving/dropping water.
6. Some guys have reported having the dealer do an
ECU idle map upgrade and solving the problem.
The GS 911 should show any coil/plug problem.
Let us know what you find out.