470 stalling during idling.

[DaleGribble]

Hi guys,
I have a 77' V20 with a Mercruiser 470,during idling after about 5 minutes the boat completely stalls and wont start for about 20 mins,had the carb rebuilt twice by reputable mechanics,new spark plugs and wires new fuel filter,fuel/water. Tanks were stabilized before winter and fresh fuel has been put in. Runs great at full throttle but need idle for crabbing,any suggestions for a rookie?