That happens when you start downloading from multiple sources, and then the fast sources drop out, for whatever reason, and leave you with the slower or firewalled sources. This can also happen if the fast source is being throttled by its ISP, or if the fast source suddenly gets busy.

A "repeat seach" can sometimes help re-find those faster sources, but the results are inconsistent.

Did you see the posts about checking to see if you can accept incoming connections? That's the best test to ensure your setup is best configured for a clear connection to the gnutella network.