Idles fine - runs great when I mash on it - stutters/stumbles with a little throttle

[CWizard]

Brent -
Your problem sounds a lot like the one I just squared away. My car ran great at any throttle postion except when just maintaining cruising speed on a level road, and there it would cut out, or hesitate noticeably. After some terribly enlightening troubleshooting I found two main problems. One - the timing advance was squirrelly. It would stick at a total advance of 34 degrees and then ramp on up (I had 17 initial + 21 mech advance, 38 total at that time). Worse, the timing would bounce up and down by as much as 4 degrees at lower rpms, and specifically in the rpm range where I would be trying to maintain cruising speed. Never found a reason for this strange timing advance behvior, but I cured it with a new distributor. Note that to find this kind of problem you have to map the rpm vs timing advance curve at about 200 rpm intervals to really know what's happening, ie, don't just rely on a point at idle and a second point when the advance is supposed to be full in.
My second problem was related to idle transition in the carb. I had adjusted the idle mixture screws (4 corners) to get max vacuum at idle and then leaned it about 1/8 turn from there. The engine did not care for that set-up, and continued to hesitate a bit while trying to maintain a constant, low load speed. So I started backing out the idle mixture screws and finally, after about a half turn out (richer) on all 4, it's pretty much ok. I also found one of the idle mixture screws was a bit rounded at the end, which certainly complicates the adjustment process, so it will be replaced soon.
Hope this is of some use to you. My carb is a Holley 4150 HP street, mech secondaries, no choke or vac advance.