AFR going lean at 6000 rpm

[Noisy]

Had the Tuscan at Surrey Rolling Road on Wednesday, the AFR seems ok for the first part of the run, but at 6000 rpm the AFR spikes from 12.7 to 14.5 then back down to 11.5 then back to the high 12's again, on the road you can feel it hold back at those revs. Anyone had similar issues? What was the cause?