The problem is not the speed of your download, - it's probably just the way LimeWire measures your bandwidth. A possible change could be, to normalize the shown bandwidth usage, but why bother?

There's nothing you could do to the LimeWire code, that would have any impact on the overall download speed.

It's also possible, that the instable speed is a result of the upload mechanism, the uploading client uses: It could be the case, that it sends you a chunk of the file, then sends chunks to the other hosts it is uploading to and after a second or two sends another chunk to you. However I think there are more important things to worry about, at the moment.