Have you defragged your HDD lately? A fragmented source file can lead to buffer underruns. Doing a lot of burning as you have done along with adding and/or deleting large files can quickly lead to a defragged HDD.
What do you mean by 'wasted discs'? Did you scan them to see what kind of burn quality they had, or would they just not be readable?
You can check your burn results with Kprobe 2.4.3 BLER scans(scanned at 4x scan speed the forum standard for comparison). You can read Interpreting PI/PO error scans to find out what these graphs mean. PI/PO is now referred to as PI/PIF. Roughly, PImax<280; PIFmax<4. Attaching disc quality scans to your post of the offending discs will help us help you.