Had an enormous amount of problems with my normally very reliable Pioneer DVR-106 producing coasters. It was writing CD-R ok but not DVD-R.
I upgraded the firmware to region free v1.08 from here and I disabled the IMAPI service. This allowed the drive to write DVD-R again but it was a lot slower.
I found that the drive had switched to PIO mode so I hunted round to try and find how to force it back to DMA and found this. Also found a hotfix what will prevent the drive slipping into PIO so easily again here.