DVD recorders all use a type of real-time writing -- it's actually slower than 1x speed recording done with a computer DVD burner. Some media actually work quite poorly at such slow speeds, with this writing style. Don't believe the myth that burning slower is a fix-all. That's not the case.
The Sony RDR-VX500 is one of Sony's early DVD recorders, and is pretty miserable overall. I'm sure that's not what you wanted to hear, but that's probably a big part of your issues. The older Sony decks were infamous among users at the time, for various reasons -- unreliable reading/burning being a common complaint. I remember testing one of those units many years ago, and the quality of the recording was inferior to most other decks available at the time, and it had issues with the known-good RW media used in the test. It's one of the few DVD recorders I've ever wanted to hit with a sledge hammer.
It's also a 6-year-old unit, probably using a slightly older DVD burner inside of it. Almost any DVD media bought in recent years is not going to be in the machine's now-ancient internal database, and it just selects a default write strategy. Most media fail miserably when burned with a default/generic write strategy. Verbatim is one of the few media that works well in this scenario, but even it is not perfect 100% of the time. I'd also note that a generic DVD-R strategy tends to work better than a generic DVD+R strategy. So use a DVD-R, if possible, not DVD+R.
Magnavox is not a very well-liked media, with many negative reports from users online. It's a CMC Magnetics discs, a company that manufactures discs often considered unreliable, found in unpopular generic brands, or discount brands like Memorex, HP and others. According to reports seen elsewhere online, CMC has also been known to cheaply sell lower-grade discs (grade B, seconds, etc) to companies that insist on impossibly lower pricing structures -- and there's always a chance Magnavox is using these lower-grade media.
I would not ever suggest a CMC disc for archival use. Only use a CMC disc for something that is quickly/easily re-copied and replaced. It's great for duplication, but terrible for the only copies of home movies transferred off VHS tapes.
If you can use a DVD-R, the Verbatim DVD-R made in Singapore, Taiwan, India or UAE -- it's printed on the package -- will be your best option. You can get great deals on the discs online, see the list of sales at http://www.digitalfaq.com/reviews/dvd-media.htm