[Bruce Edwards]

If it started after you changed the plugs put a standard set in and see if the problem goes away.
Repair shop motto. Start at the last thing the customer did before the problem.
I can not tell you how many sets of "Fancy" plugs I have replaced over the years. Bosh platinum seem to be the most at going away before they wear out. Just from what I see nothing more.
Still sounds like a lean miss to me. If changing the plugs does not help, be sure to gap them, you could try and old trick for carbs with out a choke. Plug the air filter to see if a richer mix fixes the problem. I have seen folks use an old t-shirt tied around the filter and it works to richen things up. Just an idea.
You may wish to spray a bit of carb clean around the carb base and edges of the intake to check for leaks also. If the engine speed changes you have found the problem are. BE CARFUL as some of the cleaners are VERY flamable if you go that way. Also be sure to stay away from the top of the carb as that will give a false rpm change.
Good luck and let us know what you find out.