Thanks, I don't know how long the dips have been as I have been asleep or at work. On 2 out of 6 the Arduino reset properly but on the other it just hung. The microwave, cooker , etc all reset too. The Arduino is talking to a geiger counter which draws next to nothing in comparison. I suspect it is a very short transient which does not completely drop the 5V line to zero before it comes up again. I will try different power supply first. I was just interested to see if anyone else had observed this and had a good fix.