[240racer]

I had that problem on my '89 motor too. It was because the exhaust flange gaskets were leaking and hot exhaust was blowing on the plug boots which burned them off and then they gapped to the manifold. You probably need to replace your gaskets which kinda sucks and your spark plug wires. I temporarly fixed the problem by wraping the wires in electrical tape and heat shielding, but the voltage is really high and can gap through most anything. The gasckets are cheap and it's only 8 bolts, but it's heavy and hard to work with. I also had the heat shield removed so that might have made a difference.