I only have 2 burners in this PC (Intel Core i5 750, 4GB RAM, P55 chipset, Vertex SSD, Windows 7 x64).
It copes no problem with 2 burns at 20x.
PC2 has a Intel Q9550, 8GB RAM, P45 chipset, and an Agility SSD, again running Windows 7 x64.
It also has 4 burners, and it can cope with 4 burns without problems, so it's not Win7
I'd still say it's an HDD, SATA controller, or perhaps even a badly written SATA driver.
BTW: How much CPU power is being eaten when you're burning?