burnin up!!!!
1...What is running charging voltage? Measure with engine running to simulate low-med cruise RPM, measure right at battery, and also at the ballast, check both sides and pick "high" side.
2...Is this a factory type coil or an aftermarket high output? (Wrong coil for ballast, some aftermarket coils require a SECOND ballast in series with OEM)
3...Even if factory coil, the coil could be bad, IE partially shorted.