The way torrents work is that leechers will always download from the fastest combination of seeders. It depends on the upload speed of each seeder, as well as their distance from any leecher.
If you are seeding, there is no way you can slow down the swarm, you can only contribute to it. What probably is happening is that some guy (or guys) at a university somewhere is going on and offline. He/they have what we call a "fat pipe", so they can seed at an extremely high rate of speed (fiber optic?), much faster than us home users. When they are online, count your blessings! They can seed so much faster with their high speed connection than you can, that they naturally contribute to most of the flow, reducing your contribution---this is a good thing! When they go offline, that leaves the rest of us to take up the slack, and that's when you see your upload speed take a jump.
With the current state of affairs, be glad when your upload drops---but keep seeding. Ideally, there will be at least four times as many seeders as leechers, but a couple of fat pipes can change the equation. Ratios aside, what we're shooting for is that each downloader can download at their maximum speed. As a rule of thumb, there should be more seeders than leechers.
It's simple, but confuses me the more I read my explanation, so I'll stop now.
