My Fazer 600 has started playing up when it gets to about 6-7000 revs under acceleration. Its coughing and spluttering and won't get past that rev range. Its only got 9000 miles on it and the only modification that i know of is a viper can. The carbs are balanced, the plugs seem good, air filter clear, etc. Has anyone got any ideas on this or come across similar. Also with the can, its got a removable baffle that when i put in makes the problem even worse to the point that the engine will sometimes cut out after you try and power through the coughing and spluttering bit.!! When its in neutral though it will rev round quite happily though..... odd?

This sounds like a fuel supply problem. Demand is exceeding supply due to dirt in the system somewhere but probably not the carbs as you state that the engine dies thus all the carbs have the problem. You need to strip down and clean the fuel system and fit a fuel filter.

Is the choke stuck open?