Downloads never finish...

I'm on a cable modem, running through 802.11N right now, so wirelessly connected. I moved out of the dorms and back home for the summer, but I'm rarely ever able to finish any downloads over 100mb. The progress bar goes and goes, then just jumps to 100%, even if it's still only like 90mb out of a 200mb file. Any idea what's causing this and what I can do to fix it?

I doubt this makes a difference, but I'm using firefox for all my downloads.

Vista, XP?

XP, should have specified. I'm using a Linksys WMP300N card on my PC.

Any security software, firewalls, Internet Security suites, worm protection, etc?

What happens if you use IE or something fun like LeechGet?

could possibly even be a RAM problem. I ran into this when the file would get corrupted during downloads.

there were other symptoms along with this though. CRC errors on .rars that I knew were fine, etc.