Hmmm....are you pulling data from another CD? or HD?
We had a similar problem with our 12x Burner....wouldn't burn CD reader to recorder at 8 or 10x, but fine at 4x,6x,12x.
IF a CD, I would guess that the source-CD is spinning down after sending data, but at 2x, it doesn't spin down fast enough before a read request is send...causing buffer underrun. At 1x, it sends a burst, spins down, but because it's burning data slower it has enough time to handle the delay. At 4x, it doestn' get enough time to spin down.....
My two cents.....