Usually 16x burning does the trick for me with audio CDs - you might want to invest in some Verbatim or Taiyo Yuden CDRs (you can get the TY online at www.svp.co.uk).
What drive are you burning them on, out of curiosity? The ones that don't work, I mean. It's interesting the the discs that worked were burned on an older LG drive, and a CD burner, both of which are often better for burning audio CDs.
Also...I don't have any experience of iTunes, have you tried different software?
As a last resort, the Technics might need a clean.