bodmer:
Another potential source of glitches is a switched mode power supply. I have had problems powering Arduinos with some "Wall warts" that generate quite a lot of EMC noise that gets worse depending on the grounding scheme of the rest of the circuit. On one 5V output wall wart I measured 80V (low energy though) spikes and I could feel a tingle from the voltage if I touched Earthed equipment!
I use something more sophisticated for the external power these days since discovering wall warts outputting 18v on a supposedly 12v supply.. POP!