I had the exact same problem...
I just got a Gigabyte GO-A0804a which I flashed to a Lite-on 851s. When it was still using the gigabyte firmware, I tried burning a multisession DVD in Nero and I got the same 20% buffer issue. Nero's buffer was completly empty the whole time. I find it interesting that we both got 20%. I was using a Ritek DVD+R 4x, burning (or at least trying to burn) at 4x. My system specs:
Benq Joybook 5000
Burning from a 30GB FAT32 partition with about 4 gigs free
Lite-on 851s (using an external drive enclosure to convert to USB2.0)
At first I thought it might be the drive enclosure's fault, but I burn CDR's at 40x with no problems on the same drive. I've only burned 3 DVD's and this happened on the first one, so I will keep you posted on any future mishaps. The disc I burned was using nero 6.0.0, which I upgraded. Maybe that was the problem? what did you use to burn?