Drifterxxl180

Having big problems with my KA

Here is whats done:

Head has been ported & polished , (5000 miles ago)
new head gasket
BC Cams Stage 1
headers
exhaust
SPEC Clutch and one piece driveshaft

I was driving the car hard and on they way home I hear a loud pop like a shotgun. Smoke starts rolling into the car. When I stopped to check the spark plug wire had small flame on it and the the hood had a massive dent. The spark plug wire was exploded apart and the plug was in two pieces on the #3 cylinder. Today I have replaced the wires and plugs it idled fine but upon test driving it was missing. It had also pushed the wire back up out of the valve cover. Has anyone else encountered this problem or even now what is the cause of the problem?