I've run into similar problems trying to burn on-the-fly RELIABLY to either my HP 9110i or Plextor 8432 both capable of 8x burning.
Both Afreey and Asus make decent 50x units that should work most of the time. I gave up and sprung for a Kenwood UCR-421 72x CDROM on each of my systems. Paid $90.00 for each and have not regretted it one bit. I've read about the complaints lodged against Kenwood for earlier units/models but paid no attention to it. These 72x Kenwoods are the only solution for you guys that have 12x burning units that want to burn reliably on-the-fly every single time. That is if you want to stay with IDE. If you have a SCSI adapter then Plextor is the way to go. Either way it's going to cost you a few bucks.
The Kenwoods do not spin down prematurely toward the end of a burn. My Afreey CD-2050 50x unit ($35.00) would sometimes do this giving me a coaster at the tail end of a burn, a real pisser! Plus the damn thing was noisy and sounded like an F-16! Both of my Kenwoods are quiet, fast, and extremely reliable. I have not experienced a single problem with reads; I've even burned copies of scratched audio CD's (maybe my luck).
These Kenwoods ROCK! I've copied many CDROMs (audio and data) on-the-fly without any problems. I would suggest you give it a try...
Try and find a local retailer that stocks them that has a liberal return policy (we have Fry's Electronics in my area for this purpose plus they generally have good prices); if it doesn't work then return the mutha for a cheaper unit but I don't think you will be disappointed...
Also you haven't mentioned what kind of burn you are trying to perform, audio or data. For audio it's all the more important that you use a CDROM drive capable of performing DAE (digital audio extraction) reliably and at a rate greater than what burn speed you are trying. In your case, for a 12x burn, I believe you would need a CDROM reader capable of a sustained DAE rate of 18x. There aren't many IDE units that are capable of performing high rate DAE: Asus, Afreey, Teac, and Kenwood are some of the brands that I have tried that do and are known to be acceptable. Too bad Plextor doesn't make an IDE CDROM reader but if they did it would probably cost as much as a Kenwood...
Lastly, check out this site for further help/advice:
http://209.15.164.79/
Heiny