289 stumbling at part throttle.

Give some details. What was the nasty stuff in the carb?

I tested the condenser with my Heathkit engine analyzer, model# CM-1050. Handed down from my father, circa 1960 something. And the carb was mostly just varnished up. Happened again today, seems like if I drive it all nicey-nicey it acts up, but if I drive it like I stole it, she does good. Wondering if I have something wrong in the valvetrain? I'll see if the fuel pump has a filter and change it. Totally intermittent the way this happens. Still looking for a pattern. I hate not knowing. It's just an internal combustion engine.