Believe it or not, some of that chugging is throttle control. Put the power mode in LOW and really lock your wrist in low rpm's and it can smooth out quite a bit. My Cramp Buster helps keep the throttle steady, too.
The first software update we got on the 13's in the US addressed surging at lower rpm's. Many of us noticed a difference. Did you get any updates yet?
PomGP, don't you have a pipe? Thinner air with altitude is usually just bad all around for cars and bikes. Less dense=less air per sq/in=less efficient. Thinner air only has a benefit with aircraft that cut through less dense and less turbulent air easier, requiring less power=more efficient (but lift also suffers up high). That's why airliners typically travel at 30K'. They burn less fuel, get stronger winds aloft and it's usually less turbulent.