Problems like this have existed in the past. It useally happens when the videocard and soundcard for some reason are sharing an irq. (especially with creative sblive cards ..)
Unfortunately not much drivers contain options to change the irq, so the only way is in various case to move cards to different slots. Perhaps it helps but first check if the cards are sharing an irq.
In both your cases it can also be the second pci videocard. The bandwith of the pci bus is limited and videocards consume lots of bandwith. (likely more when using the nvidia drivers instead of the nv because of more features and optimizations)