beings that you did a full OS reinstall and if you reinstalled FF, then it sounds like your ISP is limiting/breaking the connection, you are having connection Hiccups, or your Firewall Software or Router settings that are causing a Hiccup. Otherwise i would say try some remedies here:

http://kb.mozillazine.org/Unable_to_save_or_download_files

sorry i know it's an old thread but:

http://www.pcpower.com/accessories/6-to-8-pin-adapter.html

Have you considered, (since you seem to not want to build your own), something like http://www.cyberpowerpc.com ?

a custom built from them spec'd like #2 xps 720 is about $1500 w/ 3year warranty and Windows Vista etc...

I love Dell, but just a thought and an option for you. CyberPower makes good systems also including custom gaming laptops.