Your a320n may have a Philips or a Cyberdrive DVD-ROM drive. The Cyberdrive DVD-ROM drives that were available at the time of the a320n's manufacture are not compatible at all with DVD+RW media. And although Philips had many DVD+RW-compatible DVD-ROM drives, the one in your system may be a model which has trouble with DVD+RW media.
Also, which brand of DVD+RWs are you using? I hope they aren't Memorex, which gets its rewritable media mainly from manufacturers of dubious reputation. I was burned by that brand of rewritable CD-Rs, which degraded severely with each write/rewrite. So severely that some of the discs pooped out after just one write cycle. (Of course, a second rewrite appears to have been completed successfully, but when read on even a forgiving reader, constant and unrecoverable read errors developed.) The Sony and Verbatim brands of rewritable media that I used never degraded anywhere near this badly.