Anyone have any guesses as to why? It's a fairly uniform sine wave from 0 to ~150 kb/s, with a period of approximately one minute. I'm using uTorrent, and have port forwarding set up correctly. I have Vista 64 bit. My ISP is Embarq DSL. Any ideas, or any other info needed?

Thanks!

Quite normal
It depends on the other computer you are downloading from (many official sites use torrent files, but generally these files are coming from private users computers, just like yours)
What exactly are you trying to download?

It just seemed strange since I hadn't noticed it doing it on previous downloads. As far as I know, my ISP isn't doing any kind of throttling or anything. I was trying to download a couple of game demos (legitimate demos, not pirated games), thinking that using a torrent with several seeds would be faster than from the publisher's servers or other gaming websites, but on average the publisher's download is faster. I also didn't want to tie up both of my browser download allocations. (I know, I know; my bad for using IE rather than Firefox or something else!)

Just posting link: Internet Explorer: Get It Now Actually IE8 Beta, has been out for a while, hopefully going full version soon.

Nothing wrong with IE anyway, it's a youthful response to say don't use MS IE on MS Windows.
I better not get into this debate though (goods and known bads, of IE) But I wanted to say that pretty sure 80% of the world use it. (They all me must be mad!, I don't think so) I use it for Windows updates, and every time Firefox doesn't open some page. Or just normal browsing, guess what I'm fine

Anyway, with the amount of variation on speeds of P2P file sharing programs, it can never be perfectly gaged on what's better (especially seeming it changes, depending on what time of the day, you're on!)

Oh, duh! Using the uTorrent Speed Guide (Ctrl-G), had set the Connection Type at my download speed, rather than my upload speed! When I changed that it seemed to help. It's still cycling some, but it's nearly midnight so I will play with it more tomorrow after work.

Thanks for updating with extra info