when using my series 2 hard there seems to be a top end end misfire around 5.5 6k?

also just as of the this morning it was idling at 1600rpm!?

the car is due a good service so maybe this will sort it but a just wanted to check if anybody else had had this problem

engine spec

standard bottom end
stage 1 head
cvh 35 cam
radtec front mount ic
pace rad
unknown chip
full ss exhaust
20 psi

 

It's the Kent cam part of your spec that worries me.

It's most probably run a cam-lobe off.