Had similar problems myself. I have the SH-S223B with SB02 firmware and although it's a very fast burner it can be inconsistent. Regarding DL media you have two alternatives:
1) Only burn Verbatim DL media. Media codes are MKM 001 or MKM 003 or MKM 004 (this last one is pretty hard to find). They are expensive, but most writers will burn these without problems with decent results. Noone makes better DLs than Verbatim, and the pricei s indicative of the quality. They also seem to last longer than cheaper DL media, so if you're burning keepers choose Verbs if you want to maximize your chances of your data to be still readable a few years down the line.
2) If you have to use cheaper discs look no further than RICOHJPN-D01-67. These are x8 rated discs and the Sammy can burn these at their rated speed with decent results (at least the ones I'm burning at the moment, Elite M DVD+R DL spindles of 50). Managed to overspeed these at x12 by swapping media codes in the firmware, results were readable but burn quality suffers at that speed, so now I leave it at x8. My Sammie burns this media pretty fast, it manages x8 on most of the disc's surface with x6 only at the beginning and the end of the burn. Hope this helps.