I'm not sure, perhaps the problem is that this one disc is bad. Could also the performance of that particular drive is descreasing. But my bet is on the media.
It's difficult to say what is a good brand for media, since the brandname usually isn't a very good indication of the quality (the MID is). I myself almost only burn Verbatim (MID: "MCC....") at the moment, but these of course cost a bit more than you Bulkpaqs (but they are great). If you want fairly good media for a good price you should look for discs with MID "RICOH..." or "RITEK...". These are sold by Ricoh and Ritek themselves, but are also available from other brands, like BestMedia Platinum, Traxdata and many more. Have a look at this database for more information.
When trying new media I would advice to only purchase a few discs. Only buy more if you really like them.