Sounds like it's your CPU. A bigger heatsink would help, but probably not much. Same for better circulation between your case and the rest of the trunk, as hot as it is. Ideally you would ventilate the air in your trunk, but I don't know how difficult that's going to be for you. I doubt you want to be cutting vent holes in your trunk lid If you can sneak a duct into the passenger compartment, you might pipe cooler air to your case.