Nero and x12 plextor

[Sparkie]

I've been running a x12 sped plextor for a few months now with Nero 5. The problem is that the buffer keeps empties and stops it burning untill the buffers fill again. When I've done a speed test on my Hard Drive it says its 5000Kb/sec, x12 speed writes at 1800Kb/sec so can anyone shed any light onto why my buffers are struggling. I don't have anything running at the same time as I'm burning.

Please help
SImon