I experienced the same problem with my Shelby Mustang. Just as your car, it ran fine at idle and full throttle, but ran erratic at steady speeds as well as gradual acceleration. I too found dirt in the carb – once clean, car runs smooth at all RPM’s. No further problems – has been over a year since I found the problem. In my case, I can’t explain how the dirt entered the carb as I have a new gas tank and fuel lines – perhaps dirt was not completely removed at time carb was rebuilt.
Good luck,
Dan