Holley 750DP idling issue since rebuild

I have a 1974 Swinger with a 360 bored .030" and a Edlebrock intake. I have a Holley 750DP that I've rebuilt twice now. The first time, I screwed around with the setup and installed a power valve on the secondaries and opened the secondary idle a little. It ran way too rich and idled poorly.
I rebuilt it again this spring and put the power valve blank plug back on the secondaries and left them closed. It idles much smoother, not rich at all. But a new problem has crept up. After warming up the idle is good, but after driving it for a while it starts to stumble at stop lights or backing up. Then it's really hard to start again. It almost feels like it runs out of gas. I put a new fuel filter on when I rebuilt it, filled up with premium.
It has a new tank so I don't think it's anything with the fuel system, just the carb. Thoughts? Suggestions?