How do you have your spark plug cable set up. If the cable that goes from ignition module to spark plug is close to your muffler and as you increase power it tends to hit the muffler on and off that's where its comming from. That spark plug cable should not touch any part of your engine or muffler. Strap it to the cowl with a ziptie and test again. Interferance problems can be really frustrating but do one thing at a time and test before you go to the next change. Doing bunch of mods and test won't show you the exact problem.
Good Luck