Shareaza

I'm wondering, eMule for example can keep a steady upload stream until task is finished within given rules, but in Shareaza, upload speed seems to go up and down, starts uploading, drops to zero and shows "Next" and 1 second later, begins uploading again. And it keeps on doing this for everything, be it ED2K or G1/G2 or even Torrents seem totally underutilized. Why is it doing this?