It could be a variety of reasons , and I'm hoping you have gone through the stickies at the top of the forums as there are quite a few useful articles about getting the best out of Limewire.

Sometimes there are bad files planted onLimewire which quite often are in blocks and purport to come from quite a large number of hosts, often from a T3 sourse and these will sit at zero for ever.

You maybe lucky one day and unlucky the next,

Your isp might have sussed out you are using your connection for p2p and they are trying to curb it.

These are random thoughts that may or may not apply, but try a variety of files from a variety of sourses and see if you can find a pattern.

I hope this, at least, is food for thought.