How I fixed my problems with long downloads

Turns out my DD-WRT (v24 sp1) router's SPI firewall was blocking packets related to DHCP from my ISP. This caused DHCP connection to reset every 10 minutes, which stalled any downloads in progress and caused internet applications (Skype, online games) to behave oddly.

I fixed this by adding the following line to my firewall startup instructions:
iptables -I INPUT -p udp --sport 67 --dport 68 -j ACCEPT

Open router configuration page (http://10.0.0.1 for me) and go to Administration->Commands. Copy and paste the above into the "Command shell" text box, then click "Save Firewall".

From this thread.