I have a similar issue . . .
My burner burns fine at 8x, but gets significantly slower at speeds higher than 8x.
I posted a note and Pinto2 told me to check my system settings - the hard drive should be at DMA4 and the burner at at least DMA2 if I recall correctly.
Anyway, I have an eMachines and I can't change my DMA setting in the bios.
I took the drive to a friends and tried it on his and it flies right along at speeds above 8x, so I'm sure it's motherboard issue.
8x is fast enough for now - while I'm researching new motherboards!