If it burns and verifies, the disc is at least 100% readable in the drive that burnt it. Other devices may not work so well.
http://forum.imgburn.com/index.php?showtopic=12200
Check the burn quality of the discs (using CD Speed, DVDInfoPro, KProbe, Opti Drive Control), it's probably low or has problem areas.
As the OS plays no part in burn quality (it's purely a drive/firmware/media combo thing), I can't see Windows 7 / 8 making any difference.