Memorex is by far the worst brand of dvds EVER made...
The gliches and drop-offs are there because at 5 hours the the laser must write even smaller mpg2 steams on the dvd to fit the video and audio...
Since the steams are closer together on the dvd they must be precise... and bad source media (memorex) adds to the problem
Buying verbatim will solve this issue... also dvd-5s are meant to hold 2-3 Hours MAX
Also please check
http://www.firmwarehq.com/ for a update of your cd/dvd drive.... there could be a new algorithm...
(right click your cd/dvd drive in device manager to see which you have)