Idle problem

I have a problem with my idle. I have a 340 stroked to 408. The rpm in drive has been a constant 1000rpm, in park it is about 1200. I have noticed now that it surges, it will go from 1000 down to 800 and back and forth with the feeling of stalling every once in awhile.
I have cleaned the carb {on the car} made sure idle mixture screws are all the same...I am not sure what to look for next.
I have a MSD 6al box and distributer..vacuum capped off and no kick down linkage to adjust.
The car drives great, and has no performance issues while driving, only when I come upto a traffic light etc
And help would be great..thanks
AL