What about the stuttering? That's a huge list of things to try/consider. Is this a common problem? What would you check first? OBD hasn't indicated an oxygen sensor problem.
I have read some forums and read there were power issues related to head gaskets & coolant leaks when heater works only when accelerating (they said it was rare, but my heater only heats when accelerating). Would I see any other issues related to that?
Okay - the fact that it's worse when it's raining & the engine gets wet makes it sound like the secondary ignition voltage leak is it. I'll try the things you suggested. Thanks for your help.