I have to agree with mike on all three counts, it sounds like bad media, use beter media and verbatium is defanatlly good media. One thing you should do is stop thinking in terms of brands of media and start thinking in terms of media codes. Many brands of disks actually make little or none of thier media. Every blank dvd disk contains a media code that identifies the true manufacture and model of the disk (sometimes these codes can be fake but the majority of the time they are reliable). Download this free program and read the media codes and post them.
Most of the discussions here will be about what media codes work well, not what brands. When people discuss by brand, it is often because thier are very few medias that they could be. for instance, verbatium 8x + or - r found in the US is always mcc (mitsubishi). memorex on the other hand uses many diffrent medias ranging in quality so the memorex disks you got could be the best in the world or the worst. Reading the media code off of them will give you an idea (memorex also sells packs that may have mixed media codes within the same pack). Most here always avoid memorex because you never know what you are going to get.