Defrag your C: drive in Safe Mode. Then defrag your D: the regular way (i.e. not in Safe Mode). That's just the way I do it, cause I'm using Windows Defrag for now.
See how the discs burn then...and 15% free space, yowch! :eek:
Do other discs burn at their rated speed? If so, then it probably isn't a DMA issue (just trying to rule stuff out!).
Also, post your burner and firmware...those MII Verbs may be a bit iffy on your drive.