Don't completely rule out moody/fake spark plugs, it happened to me recently, a new set lasted about 10 weeks.
Because i'd changed them it totally throws you off scent.
Check the TPS as well, another common culprit that can slowly break down rather than just stop working, making you look elsewhere for trouble. Make sure the RPM needle sits quite happily on the 5000rpm mark. If it's not there's a good chance it's slowly on it's way out.
Obviously if it's out, then adjust it first, then keep checking it and see if it stays in the same place, if it doesn't then it could be time for a new one.
What about the air filter, what's the history on that as you didn't mention it?
If you do a carb balance with a dirty or stuffed up air filter then you can get a false reading which can make the bike run ropey.