Just a thought . .
My specialty is digital media, and a flag went up when I saw your numbers.
For all CD based media . .
1x = 125kB/s
therfore
15x = 1875kB/s = 1.875MB/s
If your HDD runs at 18MB/s then it's running at about 144x.
Odds are your burner problems aren't bus related. The speed ratings you've given mean that it's a new enough burner to perhaps have buffer underrun technology. If so then it should be near impossible to make a coaster unless you're working with odd DAO required images and other weirdness (not just music or computer files).
All sorts of stuff can affect burner performance . . .
HDD fragmentation
Driver problems
ASPI layer issues (for some software)
CPU load and low free system RAM
Try defragging first. If you know that's not a problem then check Device Manager to see if you've got duplicate drivers running or any strangeness like that. You may also want to check and see if you have the newest chipset drivers for your motherboard, as that sometimes weighs in on IDE performance. You might try quitting other memory resident programs and refraining from doing anything with your computer while burning. In essence, streamline your system for burning to see where the problems lie.
You may also want to check the specs on the burner to see if it's capable of speeds faster than UDMA-33. Though I'm sure it's becoming more common, last I knew most were no faster than that anyhow.
About disabling the onboard IDE controllers, there should be a setting in your CMOS/BIOS menus to disable onboard IDE by channel (primary/secondary). Some old rare MBs had jumpers onboard to disable nearly everything, including IDE, but I'm 99.9% sure you don't have this option.
I hope I haven't flooded you with stuff you've already tried or already know. I've a bit of passion about this as I have keyboard shaped impressons on my forehead to this day from the trouble my old burner gave me. I can almost guarantee you that your problem is software or driver/setting related, or more accurately not bus speed related.
Questions, corrections, etc. always welcome.
Just my two cents . . .
DJP