also check your supply voltage into the regulator it may be higher than you expect if the alternator or battery is failing.
Remember that in a simple pure resistive system a 5% increase in voltage gives a 10 % increase in power. Increase the circuit complexity and the increase in disapation will be worse because the current increase will not be linear. If Vsupply from the car is higher than expected Pd of the reg will go up increasing it's thermal input to the board. If V out of the regulator is high or unstable ( oscillating) dissapation of the processor will rise. Lack of attention to Removing heat during the design and layout of the board will ensure that the chip temperature is maximized for the dissapation. Terry