S19243H, what kind of carb do you have?

A buddy of mine had a similar issue where his motor had a "mid-throttle" lag that would disappear once the throttle was opened further. His vacuum was a little lower than normal and could not find any leaks. Called Edelbrock tech support, they mentioned that it sounded like a lean spot caused by too weak of a step up spring. They make a variety of springs that allow the rod to get pulled down into the jet based on manifold vacuum.

He installed a stiffer step-up spring and the transient lean condition disappeared.

Not sure if this is unique to Edelbrock products or if you even have an Edelbrock carb, but it sounds like you have tried a lot of stuff already...