Nevermind. Just found out how to see if the drive is DMA enabled and it is. The discs I'm using are definitely, without a doubt genuine Ritek and not fakes.
As I explained, the 16x Ridata's start off at 8x - 10x and about the middle of the disc, revert back to 4.1x as the 8x G05's do.
Any other ideas that can lead me to a solution?