How about the IAC = Idle Air Control Valve or the
Oxygen Sensor

 

Those are my next steps as well as getting rid of the Cat.

 

Changed all the plugs to NGK V-Grooves. Problem is gone. Runs great with no idle or starting issues.

 

What plugs were you using originally?

I liked the NGK v-groove plugs in my old '01 Bonneville SSEi as the engine ran better with them, but they were changed MUCH more often than the original double platinums were.