It idles well and runs OK at lower speeds but really stumbles either when you put your foot in it or almost anytime above 50 or 55 mph.
If there's a reasonably wide band of operation where it does NOT misfire, as a first-pass I'd guess ignition is in the ballpark. Timing check etc probably a good idea.
WHen you put you foot in it: is accell pump working? Does it squirt when you rotate from half to full throttle?
Clogs or bad float level could cause starvation at sustained speeds, or if it's a 4-bbl, timing of back barrels, or other problem with the other barrells.
With my Weber, I can feel the secondary open with my foot; it's therefore easy to tell when it opens.