Your story sounds painfully familiar to my experience with my Impala SS. Turned out there was a piece of grit plugging the orifice in the restrictor that is part of the ventilation system. There are two. Take them both out and look through them. Blow them out or replace them. That worthless design of an ignition system needs good ventilation to rid itself of the deadly ozone that builds up and attacks the sub-standard materials they used to build it. Can you tell I hate the OptiSpark system?