Something isn't right. With a 42/19 at 70mph you should be just around 3800 RPM. (Calculated). At 4200, you are doing around 90 mph. Did you change to a different size rear tyre? (Stock should be 170/80 15). I am running a 42/19 on an 07 model and seeing around the 3800rpm at 70 mph
With a stock engine (Thunderbike Exhaust) and a 42/19 sprocket set, anything below 3600rpm does tend to lug a bit so you need to downshift in the twisties. With a modified engine it may tend to lug at a higher RPM depending on how the torque curve shifted based on the engine mods.
