Two problems are obvious in your methods. First, it is not a good policy to burn at very slow speeds with modern media and modern drives. The firmware of the drives simply isn’t optimized for such slow burning speeds and you will get worse results. 8x is a fairly safe bet, though 12x is sometimes better on 16x media. It depends on the burner and the specific disks you are using.
Secondly, we do NOT recommend using labels. This often times causes degradation of the disks. Warping is a common problem. Sometimes you can remove the labels from the disks and get better reading performance from them…but no guarantees on that.
I personally avoid Memorex. They change suppliers so much you cannot be certain exactly what you are buying, and in my opinion, they seem to get mediocre grade disks even on mid codes that aren’t all that bad usually. (mid code=Manufacturer ID Code)
Stick with Verbatim, burn at a moderate speed and you should be ok.