I'm comming to this late and brown outs may be one of my problems, the mains here seems to be particularly dirty. Certainly I'm getting programme corruption.
I've just put a 9V alkaline battery (through a diode to prevent back feeding) onto the barrel plug as well as the custom mains PSU for the system (9v transformer 4 diodes and a 220microfarad capacitor) that is pluged into the 9V pin on the uno board. Its not elegant or a long term solution but should keep the thing alive and let me work out if its spikes or sags/brownouts that are causing the problem.
Is there a way of setting the fuse bits other than using another arduino??
Conversely does anybody know about the effects of voltage spikes on the 328 chip