Missed writing

I have bought a new liteon 40125S that i’veflashed to 48125W and i’m having error while burning, i’m using nero and the used buffer would sometimes get to 0 then i get an error message saying the cd wasn’t burnt. I’ve used the test to check the max speed and it seems that i can’t go past 40x (even though that i’m reading from the hdd)
Any help? Shouldn’t the buffer underrun protection prevent this kind of problem?