Micro controller reset due to noise, PCB design for noise immunity
In the AVR Appnote on hardware design considerations on page 5:


  To protect the RESET line further from noise, it is an advantage to
  connect a capacitor from the RESET pin to ground. This is not directly
  required since the AVR internally have a low-pass filter to eliminate
  spikes and noise that could cause reset. Applying an extra capacitor
  is thus an additional protection. However, note that this capacitor
  cannot be present if DebugWIRE or PDI is used. If not using High
  Voltage Programming it is recommended to add an ESD protecting diode
  from RESET to Vcc, since this is not internally provided due to High
  Voltage Programming. Alternatively, or in addition, a Zener diode can
  be used to limit the RESET voltage relative to GND. The Zener diode is
  highly recommended in noisy environments. The components should be
  located physically close to the RESET pin of the AVR. Figure 2-1 shows
  the recommended circuit on the RESET line.


I suggest you try an external capacitor of 0.1uF to ground, and a schottky diode to from RESET to VCC to act as an ESD clamp. Add a 5.1V Zener from RESET to GND, which comes highly recommended in the manufacturer's text.