I have an ´82 Yamaha xt 550 which is plagued by a hard to find mid-range top end miss,Good mechanic working on it has replaced all parts likely to cause(appears electrical)Ignit.stator,CDI,coil&HT lead,source&pick ups all checked ok,XT is in mint condition,fuel system clean,cleaned carbs etc. can anyone shed any light or had a simmilar experience?