Intermittent Speed Drops On Downloads?

I have two computers, both of them running Windows 7. One of them has been experiencing an odd problem for about six months now, and now that school is out I have time to mess with it. When I try to do certain things, the download speed periodically drops to zero. Specifically, I've noticed it when using Firefox and Steam, but also for IE and a few other browsers I installed for the sake of testing. I know I'm not *losing* connectivity, because everything that isn't specifically downloading a file works fine. Skype calls work fine, Utorrent streams at a steady rate, but for some reason Steam and Firefox have speed graphs that look like the heart rate monitor of someone with a very weak pulse. It starts off strong, a nice solid 1 MB per second, and then after 10-30 seconds it'll slowly decline to 0, and sit there for anywhere from 10 seconds to half an hour before picking up to full speed again. I've tried reinstalling drivers, I've tried buying a new network card, I've tried running antivirus scans, and I've tried uninstalling my antiviruses. The only thing that works for sure is putting it into safe mode. In safe mode, the downloads proceed smoothly. I tried making a list of everything that ran in safe mode, and then rebooted into normal mode and closed each nonessential process and service, but that didn't work either.