burning plugs

[bobcat68rs]

I have a 440 that is burning the electrode out of the #7. #5 is showing signs of it starting to burn it has little grapes forming on the electrode and strap.
I have put a new harness in the car also new electronic dist, plugs/wirer's. It was running good at first then burned the plug? plug gap's 40. timing set @10 adv. total timing 36. After it burned the second plug it has developed a slight miss back thru the carb even with new plug. Any thoughts?