really hard to tell, you need to start eliminating stuff. What does the ignition components look like? Old and worn, or decent and clean. I thought some of those had issues with cylinder heads, but I could be wrong. It really doesn't sound like any major fuel components since they typically wouldn't run great either, it kind of sounds like something is bleeding off fuel pressure which I think is where Craig was coming from. Using a fuel pressure gauge to see how fast fuel pressure drops off and comes back after shut down and upon starting might be a good test to do and see how that goes.