Harsh Environment MCU Power Supply
One way to "bullet proof" your power supply is to add transient voltage suppresors (TVS). See this application note for instance: http://www.vishay.com/docs/88490/tvs.pdf‎

Also check the fuse configuration of the ATmega32. Some of them may help to make your system more robust. In particular, check the Brown-out detection module and the start-up time (SUT0 and SUT1 fuses).