From: Vancouver, B.C. Canada
It sounds to me like those are cruddy CDs. Just a guess, I've never heard of the brand, and don't know what factory makes them. Sometimes low quality CDs have a hard time being burned at their official burning speed...also, sometimes shady dealers put 16x stickers on 8x CD-Rs. Verbatim is a very good CD brand, most of their CDs are made by CMC (maybe just their newer ones, I'm a little unsure of that one). My suggestion to you would be to find a place that offers to replace defective media that you buy from them. Most stores won't do it (especially radio shack or future shop!), but some places will. If you are local to B.C. or Alberta in Canada, you might want to check out my store eNovationMedia, we replace CDs on a 1:1 basis, if they're defective. and CDs that only burn at 2/3 their max burning speed deffinatly sounds defective! good luck!
Canada; Leading the world in being just north of the U.S.A.