My suburban periodically idles too low when i come to a stop.......

my 92 chev suburban, 350 auto, 4wd, 50m on rebuilt motor & trans, will frequently, but not always drop to an idle way too low at a stop sign or light.  It almost dies and the truck shakes quite a bit.  With brake pedal depressed, the motor will not immediately respond to a tap on the throttle,  still shakes.  Upon taking off, there will be a delay in throttle response and then the motor engages and off we go.  This does not happen all the time just about 1 in every 15 to 20 stops. The motor runs excellent all other times and has no problem starting. I've replaced fuel and air filters and used fuel and carb treatments. Any help with a diagnosis would be greatly sppreciated.  Thank you.