If you look through these forums most people will tell you to used only verbatim for DL media burning. Why? because they are consistant on there quality. The problem with Ritek is that it can be hit and miss with the quality. So it's something to take into consideration when purchasing DL media.
Your media should have a max burning speed on them 2.4x to ???.
I would bet that it's the media at fault here.
You could try updating your firmware.
Try a faster speed IE 6x maybe 8x depends on what your media is rated for.