I know you're done, but if you didnt do it, next time uninstall and delete drivers/files related to the nic and then full reinstall of them. So much gets bound and configured that sometimes you cant just fix a bad network issue with an existing installed nic and all the files.
Since installing a new card fixed it, the issue might have been just a corrupt configuration that could not be fixed without ripping out the drivers and any nic utilities and reinstalling them to let Windows do all its protocol and stack binding/configuring magic.
That has been the fix for me at times, just putting that as a possibility.