I have come across 2 incidents of i7 based motherboards recently from different manufacturers that have sometimes powered up OK but more often than not gone into a 6 second repeat cycle of 'on' for ~650ms then down for ~5.5 secs. All power rails appear to be stabilized during the initial switch-on period and the 'power OK' is at a solid level for ~500ms. The 'power switch' signal from the motherboard is then removed and the supply shuts down, with the cycle repeating 5.5 secs later.
I have seen other discussions about this on the internet but no explanation of the cause. Does anyone have any suggestions?