I'm using Sony DVD-R, Platinum DVD-R (= Ritek G05) and RiData DVD-R (also Ritek G05). They had burned fine until last night, but all of a sudden this morning they didn't.
In the meantime I've upgraded to Nero 184.108.40.206 (legally, what a shame), and the same behaviour occurs. After updating to FW 1.04u, 6x seems to be possible (running at the moment, but after about 20 % the speed drops to 4x again.
This is ****ing me off.