Bean,
Did it ever burn data DVD-Rs properly? If it did, think of when it started to act funny.....
Here are a few things to try: firmware upgrade, burn slower (I always burn essential data files at 4X or less), exit all non essential programs, use better media (Taiyo Yuden, Made in Japan, id code TYG01 or TYG02), defrag your drive (only if your video is burning properly and on a differnt partition than your music data files).
That Lite-On SOHW-1633S is supposedly a decent burner. FWIW, I run Nero Ultra v6.3 and have no issues even at 12X with my Pioneer DVR-108 via IEEE 1394. Never burned a coaster and it has burned easily 70 plus DVD-Rs so far. There is a distinct possibility that the burner may be going IF you have sucessfully burned DVD-R data discs in the pst without glitches.
Good luck,
ANDY