One thing you mite check is the ground strap that hooks to the coil under the cap. I have seen a lot of people with the same problem when it can be as simple as a bad ground.if your hei has been worked on it could be the strap was not even put back on, or the black wire is broken.The engine will still run but will miss at high rpm or under a big load.And if this is the problem, check the leads to the winding on the coil they are probably melted and could short on the field frame.Jeff