"With these daul fan PSU's I presume I need a vent in the top of my case for the 'top' fan ?"
Probably not - check your case.
"....monitoring software....real time graph...12 volt supply is steady."
It probably will be, until the PSU can't cope with the blips/loads anymore and the "power good" line is pulled low triggering a reset.
Keep us informed of developments pls.