If it happens on other mobos then it sounds like the card itself. There's no hard drive boot until the bios hands over control, meaning it can't be a driver issue.... could be tarnished contacts, have you checked them, try wiping them with a clean dry cloth - do not use any solvent, cleaners or fluid on them.
The only common thing to all the PC's is the card, very unfortunate if all the mobos were faulty highly unlikely but not an impossibility.
In my 'telly' servicing day's I've had PCB cards where the edge connector contacts have become faulty, they can 'creep', they are only copper tracks that are either tinned, silver or gold plated..... these are rather soft, the pressure of the socket contacts can actually cause them to 'pit' causing faulty connections, heat could tarnish them too.
The contacts should look rather shiny and free from wear maybe they are worth a look.