I have the same problem with my two pc;
on pc1 whenever another program touch the disk, es. opening an avi, the disk start to seek heavily and the buffer slowly empty then after a minute it reach 0% and the writing stop momentarily to fill the buffer, then restart writing and everything go fine untile another program just touch the disk, the it start to seek heavily for a minute until the buffer is totally empty; this mean that i can't touch the pc until it finish the work, i write dvd at 4x or 6x.
it's a quadcore with 4gb/ram xp_x64 and sata disk, copy files disk1 to disk2 reach 80mb/s , copy files to the same disk reach 30mb/s, with nero and two usb burner usb i can write 3 dvd with different data at 4x without underrun. cdspeed give 90 to 98% quality on scans, the writer it's a liteon 20a1 ata, the disks are seagate 500gb sata and hitachi 500gb sata connected to ich9
the second pc has a normal p4 and a normal xp, in this one writing dvd at 4x i often get two to four underruns
when i start writing the buffer slowly start to go down and after 30 secs reach 0% and it remain at 0% for minutes (it happended also that i didn't had underruns so i had the entire disk written with the buffer at 0%), so i usually see the writer buffer changing a lot.
copy files to the same disk reach 25mb/s over network it reach 20mb/s (firewire ethernet), didn't tested wih nero, i always write at 4x with a Nec 7173 sata , disk is seagate 500gb connected to ich5, defrag scheduled every night. previously i had a 250 pata disk and it was the same.
since it didn't seem a udma issue, antivirus, any tools etc, i can't understand what's goin on...
edit: i didn't reported this in the post; the average verify speed goes from 11x to 13x that it's very good....