Hello fellow sufferers,
Logging mains showed no abnormalities except em noise in the UHF band when LED lights are switched on. This does not appear to affect mains supply to devices but messes up bandwidth of PowerLAN. My iMac 27 problem was eventually fixed by Apple replacing logic board, PSU and almost all other internal parts in the Apple store. Since then the iMac has been behaving perfectly. My conclusion is that there must be some electronic components in the iMac 27 which can fail leading to the machine shutting down. Nothing else has changed in my house, all other Macs work fine, there are no spikes on the mains, this is an Apple manufacturing problem and it would be nice if Apple could acknowledge this and fix it. Sorry I cannot provide a magic bullet to fix this.