I got a 350, in a built rig, my van, that has an Edelbrock Carb on it. I chased the same symptoms you have described for almost a year. I even called Edelbrock and they e-mailed me an off the wall schedule for adjusting the IMS. They told me to go 2 1/2 turns from bottom to begin adjusting. I later tore the carb down and found a crack from casting through the carb.
With all fairness to Edelbrock, I did find some vaccum leaks throughout. Their manifold was machined with a very larg gap that wouldn't seal with a gasket. I got it machined. The vaccum port by the distributor was very sloppy. Even with the fitting tight, it would wobble. I had to drill it out and retap.
I found, the gasket surface to the carb was not true. Heavy vaccum leaks.
The rear vaccum tap was very loose.
The intake planes were off.
Everybody has their off day. Edelbrock is one of the first after market sources I turned to untill now. Even their tech had an hair day on this. They would not accept a return as I had taken a year to find the faults. It was in my shop show van and was in daily service.
I had to get another shop vehicle. I just got burned out on it. I also had a machine shop do the rebuild, and should have done it myself. The things I found are things I check. Finding these things, I now doubt the rebuild.
I chased the exhaust, the timing, valve adjustment. I had to replace a lifter, brand new Edelbrock. I was in the engine daily to just get to the shop.
It sounds like vaccum, and I hope it is easier to find than mine was.