How can I fix my abysmally small download speed?

Recently, my download/upload speeds have dropped to nearly nonexistent levels. Before, I was downloading at about 300 kbps-1 mbps, but recently the fastest I can get is around 6 to 8 kbps. This is NOT because of my ISP (I use Clearwire, not Clear), and my internet still functions like I'm at 300 kbps for displaying pages. When I open a download (I use Firefox), the download speed starts at around the previous speeds (~1 mbps), but gradually drops by about 10 kbps every 2 to 3 seconds until it hits the 6-7 kbps mark. I highly suspect that this is because of my antivirus programs (I have a few: Bitdefender, Symantec antivirus, and Cisco clean system agent, and the basic windows firewall). I've tried disabling the firewalls and antiviruses, but the download speed only raises by about 1 to 2 kbps. However, I can "distract" whatever is slowing my download speeds by opening several downloads or internet-related programs, which raises the download speed significantly until the program finishes loading, at which point the speed drops again. I've been scouring the internet and google for help, but haven't found any so far. Any help/advice is greatly appreciated. Thank you!