If you can, do a test burn in Nero CD/DVD Speed (http://www.cdspeed2000.com ) and see if it really Is your drive. I have no clue as to why, but there is a serious bottleneck somewhere, and this will make sure it's not the drive itself. To test the drive's recording capabilites in CD/DVD Speed, click -Run Test>>Create Data Disc- and wait for it to finish.
If a similar thing happens in CD Speed (plateau, drop, plateau, drop, etc.), then it's your drive. If not, then something isn't configured right.
-I haven't used Alcohol, so I don't know if you can raise/lower the buffer size (Nero Burning ROM doesn't seem to like anything larger that an 80MB buffer, or nothing more than 40% of your physical memory, whichever is lower).
-Is it possible you don't have enough free memory to support a 128 MB buffer? For a 128MB buffer to fall down so often when using ATA100-133/8MB buffer/7.2K RPM hard drives is ridiculous, provided there is enough physical memory available (possible intereference of the Windows page file?).
-Honestly, I don't know where the problem would come in unless your hard drive is fragmented or something is (for some reason) trying to access it at the same time. Besides the page file, there aren't many things that will access your drive when using a lot of physical memory.
If the same effect doesn't occur with CD Speed, and your hard drive is indeed defragmented, then you might want to benchmark various parts of your system to give you an idea of where the problem might reside. Report you results, please.