If you bought the two spindles at the same time, from the same place, chances are they are both the same batch, and from the same manufacturer. Which means, if one is bad, the other is likely to be too.
I don’t touch Memorex blank media. Period
Download Nero CD/DVD Speed. (free) http://www.cdspeed2000.com/
Load a blank dvd into your burner, then start this program. On the opening page, near the top, your burner make and model will be listed, and next to it will be the firmware revision that you are currently using. Click on the Disc Info tab, and tell us the MID code of the blank dvd…this is the manufacturer id code.
Verbatim 16x or Taiyo Yuden 8x +R are the favorite blank media around here. Verbatim can be found locally fairly easily. TY can be found online at sites like rima.com and supermediastore.com…make sure to get the TY 8x +R and nothing else.
Your problems do sound like poor media is to blame.