I finally found some time to continue testing. I'm still not sure what caused my earlier problems with DL, but the only cause i was able to narrow it down to seems to be the fact that different software was trying to access the drive at the same time (notably Plextools, CD-DVD-Speed and ImgBurn running at the same time). Interestingly this has not been an issue with SL media and never an issue either with any other drive i used. Maybe there is a problem with software being able to 'lock' the drive while using it?