OK, it would seem that something is definitely screwy between your NIC and your router, because according to what you stated, if you by pass the router the network connection works as expected. So have you tried
1. a factory reset on the router, to purge any corrupt route table information
2. examine the mac address of your NIC, I have heard of a missing mac address causing similar problems
3. compare your mac address with the mac address of the router make sure they are both unique.
4. check your router for any type of ip filtering, mac filtering, etc. although I would suspect that would prevent it from working at all.
5. you are not running any type of third party software like zone alarm, comodo, or have not on this clean install, installed Norton's or McAfee's or anything like that, that may be firewalling.
6. make sure your ip addressing is unique but similar to other network nodes. subnet, default gateway, dns servers etc
7. ping another computer on the network....same packet loss??
8. install a different nic and if this one's embedded disable it in the MoBo bios, although if it works fine when connect to the router, then I don't know what good that would do.
9. did you ever use the "Clone MAC address" feature in your router from this computer?
Just grasping at straws now