It might be hard to believe but only Verbatim DVD+R DLs give consistently good results (don't get the Made in India ones).
If you can't return your Memorex discs or exchange them for Verbatims, try these steps (but don't hold me responsible for the burn quality on those discs ):
make sure your burner is using the latest firmware available (in ImgBurn go to Tools/Drive/Check For Firmware Upgrade);
try different burning speeds (2.4x is preferred);
try burning with a different burner;
always have the Verify option enabled, as it ensures that the disc was properly burnt.
As for the layer break you don't need to worry about that: if it needs, ImgBurn will ask you where to put it; in all other cases it's calculated automatically.