235 engine problem

[cybermca]

hey guys,
i have a 1955 chevy truck with the stock straight six. i did some basic overhauling on it when it was purchased a few months ago and got it to run quite smoothly. it sounds like my grandma's sewing machine. my problem is when it is warm and has been driven around some, it won't start sometimes. i have done the following,:
new battery, new wires,plugs, cap , rebuilt rochester carb, new seals on all manifolds, rebuilt starter, rebuilt generator.
i am trying to figure out what i have missed. it could be a combo of weak spark and fuel problems, but they both work 80% of the time. no vacuum leaks and all new wiring up to the coil. huh????
any help............