Eladar
There are a few things that can cause the symptoms you are describing. First some DVD players, especially older ones, have older firmware that have a difficult time decoding different types of DVD media. i.e. -R. +R, +-R, RW types, duel layer, ect Each DVD player is different and require different sets of firmware. Most of the time this firmware is not easily upgraded, but can be upgraded. You should be able to go online to see how your standalone DVD player is upgraded.
The firmware decodes a binary image that is created by your DVD player's laser which brings us to another consideration. If you laser gets weak or misaligned, it could have a hard time reading specific types of media while having no problems at all with others. While this is not typical it can still happen and generally a sign your laser is going to fail.
Also, the speed at which foxy burns the DVDs has a great effect on the quality at which you DVD play can read the disk. Because time is money most companies burn at a high rate of speed. This produces a higher rate of failures but the failure rate is still rare and giving the customer a new DVD is much cheaper than verifying each DVD at pre-ship.
I don't know what media foxy uses but some smaller companys will a codec known as SVCD or KVCD. This allows the use of a CD to be played in a standalone DVD player. I don't think foxy would put out poor quality like this but alot of illegally sold software is typically done this way. While I was in Afghanistan almost every illeagal movie disk I purchase was encoded on a CD.
(although only illeagal if I broght them home....ummm...which I didn't)
I'm not sure if this helps but thought I would let you know.