Burned plug wires.

I'm wondering if your headers are creating excessive heat.
Is this happening in competition?
Or is this a street-engine?
Are you running a Vcan?
A streeter IMO, HAS to have a v-can. The power timing is only the max WOT timing. For ALL other load and rpm settings, without a Vcan, the timing will be retarded. When this happens the mixture finishes it's burn in the headers.And they get very ,very, hot.
If this is a street engine, Ima thinking your ignition timing is late.