My car always gets hotter running in the garage, even with all the doors open. Airflow isn't as good as when you're in free air... My place in Vegas, I could watch the temp creep up another 10 degrees if I let it idle in the garage before shut down (after the commute home) - let it roll back to the driveway and it would creep down again.
The ballast resistor is reducing battery voltage by burning it off as heat, so it gets hot. It's basically like a 40+w bulb that makes no light (hopefully). In case you're not familiar, the ballast is intended to make ignition voltage the same while running and while cranking. Battery volts drop when the engine is cranking so the coil is designed for a hot spark at 9-10v, but then once running the system voltage climbs to 14+, so the ballast is used as a voltage divider to drop the coil volts back down to prevent it burning out from excess current.
Too bad it didn't act up... Might take laps around the neighborhood until it does?