The link below explains the 'kernel taint' nicely...

www.linuxforums.org/.../...odule-taint-kernel.html

... basically it's impossible to debug errors if proprietary modules
are installed that there's no source code for. Sorta ties in with that
other long-running thread in here.....

As for the same card working fine in the other PC - even if they have
identical hardware in all other respects it's possible for one of them
to be less stable than the other. As hardware is pushed to greater
levels then the vagaries of manufacturing can have greater effects.
Only way to be totally sure if it's the card would be to swap the two
over - if the instability stays then it's either software or other
hardware at fault but if the fault moves to the other PC then it's the
card itself at fault.

Intermittent faults are the pits to track down unfortunately and in
many cases there's no actual 'fault' - it's just tolerances in parts
being a shade too far apart somewhere :-( Not reassuring I know and
you've already done a lot of the troubleshooting with no success.

FWIW I fully expect you'd have the same issue if using Windows but
your supplier is bound to pick on Ubuntu as it's an unknown for them.
Any chance installing a Windows OS to test this theory out ??