Two things come to mind. As much as I love Holleys, the one thing that has plagued them in the last 20 years is the metering block being slightly lean as the fuel expending from idle to big throttle opening is lacking. I am really pulling out some dead brain cells here because something about what you are saying about when this problem started. Valve springs. More common than you think. Every manufacturer has tried very hard to stay on top of this on the BBC engines and it has been a tough tough world to be consistant and succsseful in this area. I can recall on more than two hands and both feet where this was the problem (I had to stop because if if the number goes any higher I just doze off) give it a thought. I find myself frustrated offering my "2 cents" worth throught the internet, I like the hands on approach for diagnosis.