HEI likes .5 ohm coil. Running a .7 is stretching it, if you have included a ballast resistor, you are stretching it even more. HEI will not operate near optimum with .7 + ohms. This may or may not be your issue, but I am just putting it out there.
More likely you have power, grounding issues or EMI issues.
Are you powering your coil circuit and HEI with a good relay? Low current feed could be part of your problem.