I stop at a light or at a stop sign it starts having bouncy RPM's?

I have the car listed above GL. 3.0 Liter engine V6. My problem started about a year ago intermittently and has gotten worse. First start of the day it starts right up stalls and then I restart it and it stays running and runs fine. Car runs fine while driving but as soon as I stop at a light or at a stop sign it starts having bouncy RPM's (like up and down up and down). Sometimes it fixes itself and sometimes it drops so low the car stalls and I can restart it fine. If I can catch it in time I can usually throw it in park really quick and it kicks up the idle or RPM's to that of a parked running vehicle and it stays running fine with no problems. Catalytic Converters were checked and the rear one was plugged solid and both were replaced but car still does the same thing. Any suggestions.