hi my name is capt. mike I,ve been a mechinc for over 16 years and i live on a 33 cheoy lee in beaufort sc. i have another possialbity for you you say you have gone over your fuel system and it sounds like you have takin care of your ignition to. buthave you check compresstion when the motor quits. what can happen is when a motor gets some hours on it the valves can stetch causing you to lose valve clearance. the eazyest way to fix this is to get the motor hot then remove the valve cover from the side of the block and check the valve clearance you will no right away if that is the problem. the idea of have this clearance is it gives the valve a sertan amount of time to rest on the block to cool and keeps it from burning up. if this is your problem you might find that you will need to replace a couple of them the others you can get away with grinding a little bet of of the stem. i don't know if this will help you with out seeing the motor my self but the next time you go for a long sail bring your compresstion gauge. let me know how it goes

Capt. Mike