I would update the drive's firmware. Although I would think that the original firmware would be able to handle your discs, updating the firmware can greatly improve performance (especially true with NECs). Going here, I would suggest using one of the latest versions of Liggy and Dee's modified firmware, probably Dee's 2.U5 - http://liggydee.cdfreaks.com/page/en/NEC-ND-3520A/ Download the file and follow the instructions, flashing your 3520's firmware.
The discs that you have, assuming that they are real Mitsubishi discs and not some kind of counterfeit media, should be of good quality. I would suggest burning them at 8x and I would also suggest trying another burning program if your current one does not allow adjustments in burn speed (there may be a place to adjust it in the settings, I don't know). There are a couple of freeware programs that would work, ImgBurn and CD Burner XP Pro. ImgBurn is commonly used and is mostly used for burning .ISO image files, but also has some support for burning standard data files, although CD Burner XP Pro might be easier to use in that regard.