Dump the gas. Even with new gas on top, the stuff has a lot of precip. Try running a few ounces of Yamaha carb clearer into them, let them sit an hour or so, then drain and fill the tank with fresh gas. Oftime the bike will run with old gas until you put a load on it (sounds like what is happening) This MAY cure the problem, but chances are the carbs are munged up.
You didn't say how many miles are on the bike, and how many miles are on the plugs. If you only ran the bike at high idle and no load in the garage since OCTOBER, pretty good chance the plugs are all sooted up, too.
If new gas, some Yamaha carb cleaner and new plugs fix most of the issues but it's still a little balky, try a few tankfuls of Chevron, or buy some Techron from your local auto parts store. This stuff works, as long as the carb orifaces and channels aren't COMPLETELY plugged. Couple tankfuls of gas with Techron really does make stored bikes run better.
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