I'm be wondering why your 8x DL media can only be burned by your burner at 2.4x. I'd suggest a check for firmware upgrades but it seems yours is already up to date.
We do recommend the MKM-001s (which are rated at the lower speed and are the ones with which you had success).
As for the settings, go to Tools-settings and click on Defaults. Answer Yes. And don't muck around in there again!
At the very least reduce your buffer from 256Mb back to 40Mb.
As for your troubles going away, you might just have a bad (batch of) disks, or the burner doesn't like the MKM-003s.
Regards