Is your bike sitting in the sun and getting the gas tank hot? The bubbling sounds like the fuel is getting hot and the light ends are boiling off, changing spark plugs will not stop a fuel problem.
You might try a different brand of gas. Keep your bike out of the sun. One more thing make sure your gas cap is venting, I haven't ran across one that doesn't yet, most will leak fuel out of the cap.
If you are using gas with the alcohol in it you might need to see if you can get gas with out the alcohol premium gas should not contain the alcohol but in these days and times its going to be harder and harder to find good fuel that had no alcohol.
Norman