Misfire problem or Vtec issue

[Gb1970]

Hi
I run a dc5 race car in the uk, we have recently gained a problem where the engine is running lean above 7000rpm causing it to misfire....
Any suggestion to why would be good to add to my potential issues !

[Spunkster]

Without the calibration and a datalog and knowing what has been done to the car, it is really hard to speculate.