Hi, I keep getting dirty disc warnings when I try and record to DVD-RW's with my Sony RDR-GX7. If I use a brand new disc like a TDK, out of a sealed box, i get a dirty disc warning after about 1 hour of recording at SP speed. If I then take the DVD-RW out and look at it, it looks total clean, no dust or finger prints. This has happened about 5 times now with different discs. Why do I keep getting these errors?