help on the 69 before i burn it.lol

[72shadetree]

on the 69 it will run good for about 15min then it seems like it loses 4cyl.and back fires.when it dies it will fire backup after 10/15 min. and run decent for a few miles
after dark on the plugs the porcelin part that you can see with the wire on it glows blue.its not arcing or jumping it looks life a blue light that gets brighter the more you rev it up
been thru tank and fuel lines
new filter
new coil/cap/wires/plugs
swapped dizzy by the way its a hei
its getting good spark

please help