It could be a progression of lens failure or just getting dirty/dusty. Burning disks at 1x is actually not as good of an idea as you might think unless you are using a very slow computer with not much memory. The data will actually flow at a much better rate at 2x-4x in most cases. Make sure nothing is running in the background and restart the computer before buning to refresh the memory. Then at least you can rule that out.