Running fast so will make this short and to the point.
Possible causes:
Worn or improperly adjusted linkage and or Heim joints.
Worn bushing in the primary where the foot shift linkage goes through.
Shift Pawl adjuster screw not adjusted properly. This is the adjuster screw right next to the shift arm on the tranny. It must be set so the pressure during travel is egual on both sides and the lever centers. This is a critical adjustment and has a huge effect on shifting.
Bent shift forks. Since you mentioned banging gears at 6k, you may have bent the shift forks. This is real common on the FL models with heel / toe shifter because there is a tendency to stomp down on the upshift and this puts a huge strain on the shift forks. If you use the heel to upshift do not use it when speed shifting!!!! That heel can exert a lot more force than your toe does.
Other possibilities, lazy foot, not getting full clutch release, worn shifter drum and some others.
Time for a service manual and some checking.
