I have a 256K/128K DSL connection, but it seems that most of my downloads come only at 4KB/s (unless downloading from multiple hosts). Also, they shift between the normal (2-14KB/s) download speed every other second, and the rest of the time 0 KB/s.

My uploads don't work at full speed either. Most of them do only 2KB/s, but at their best, they can be up to 8KB/s.

I am almost always connected to about 4 supernodes, that should not use my bandwidth too much.

How come doesn't my bandwidth be fully utilized?

I've had the same experiences. The reasons for the slow downloads are the host your downloading from lied about his connection, limited his upload speed, and/or alot of other people are downloading from him at the same time. I've had downloads that never get above 2KB/s or 3KB/s and I've gotten lucky and had other downloads that where 50+KB/s.

As for the upload speed that has to do with how your set up. I set the upload slider to use about half of my 128k so I can still surf the web or play games online no problem. This is about 10KB/s of upload. At first when I started using Limewire I left it at the default of 5 upload slots and each slot was only getting 2KB/s( 10KB/s / 5 upload slots verifies that this is working right ). I have since changed it to 3 upload slots, there are fewer people downloading from me but they get there downloads faster.