In every one of the cases I have seen of this behavior the problem is an aftermarket fuel filter. The "micro" filter held in the sediment bowl with a little spring seems to be the worst, followed by one that has been inserted in the middle of the gas line between bowl and carb. For some unknown reason, they seem to work ok for some people but not for others. My theory is that they work for people whose system is so clean that they don't really need them. Remove any extra fuel filters is the first thing to try for fuel starvation, which sure sounds like your problem.
Here's a picture of all the fiilters I've tried. The red fiberglass worked the best but it's been replaced by a paper model that does not work for me.
