50? Thats a lot of discs!
Unfortunately Memorex may not've been the best brand to have bought.
It's not that they are a poor brand full stop, but rather they are very inconsistant. I have had Memorex branded CMC single layer discs that varied wildly from spindle to spindle. One batch would give excelent results; 3 PIF max, 400 PIF total with a perfect read back curve, the next spindle 10 out of 25 were coasters.
Have you done a TRT on these discs? Were there any slowdowns coresponding to the position of the high PIF peaks on the Q scans? I've had worse results than this with some cheep CMC MAG D01's (RiDisc) on my 1650 and they are perfectly usable in all three of my standalones.
I would try a burn or two in both your NEC drives to compare, they may do a better job. Update to the latest firmware from NEC or even better visit Liggy and Dee's site for their riplock removed RPC1 firmware. If the NEC is still under warranty beware that using modded firmware will void it.
If you try with your NECs post the scans in the NEC forum. It would be interesting to see the results!