Try to get some Verbatim dual layer media. It's generally considered to be the best. Dual layer media is really hard to manufacture, unfortunately, and few are getting it right on a reliable basis.
If those discs work, you have an answer... it's media issues. Those discs are the most likely to work right.
Don't necessarily burn at the fastest speed available. Burning at a speed or two below maximum often increases quality. Don't go to the slowest speed though as most burners do best somewhere in the middle. For example, if you have 8x DL media and your burner supports 8x, try 4x.