While we are on the subject of how harsh this environment is for electronics may I suggest LM2940T-5.0 as a replacement voltage regulator with the proper filtering. I too also agree with opto's maybe 4n35 or at the very least a transistors on the input. 3 or 4 bucks extra will go a long way in possibly saving to replace Atmega etc, just my 3.26 cents
http://www.national.com/ds/LM/LM2940.pdf