First question is where did you source replacement carb from? Genuine or aftermarket?
Have had issues of the choke binding up and/or jet block being drilled improperly.
I would do is change the plug again
I just fixed a victa that had an intermittent issue with similar symptoms- the barb on the tank was not moulded right and had a little thin whisp of plastic intermittently blocking flow. Somehow it managed to run fine for 5 years and only then start to show symptoms of surging and spluttering
