Verbatim 8x DVD+R DL blanks probably didn't exist at the time that burner was sold, so it doesn't know how to burn those blanks, and the generic 2.4x write strategy doesn't work either.
Your system seems fairly recent so I'll assume it has at least one SATA port available, so the best burner you can get is a new Optiarc (AD-7240S / AD-7260S or newer will be fine).