Buffer under-run and unreadable discs are often 2 unrelated issues. However, some CD burners, especially older ones, do not handle under-runs well and the burns will fail or the discs can be unreadable.
Problem 1: Buffer under-run is due to your hard drive being too slow for the burning task.
Solutions: Lower your burn speed; increase the size of your read buffer; avoid using the PC at all during the burn; confirm that ALL drives are in UDMA mode, not PIO; defrag the hard drive with a proper dedicated defrag tool; if you're burning lots of small files, consider creating an ISO 1st, then burning that. Or use the burning software's option to cache all the files.