In Alcohol, the "Memory Buffer" is the read buffer, not the drive buffer. So if it's dropping all the way down, then your system is not delivering the data fast enough. If you want to watch both the drive buffer and read buffer, use Nero to burn your images.
You can set the read buffer size in Alcohol recording options. Try setting it larger if you cant find the cause of your slow transfer rates. This assumes you have adequate free RAM. If your buffer is larger than the free RAM, it will force paging file use, which could cause a slowdown.