Are you using the drivers and software that came with the card or the ones built-in to Windows? If you're using the stuff that came with the card, uninstall all of it and follow the Windows prompts, ignoring the instructions that came with the card. If you're using the Windows drivers, try following the instructions that came with the card.

I've had to go to a buddy's house three times due to issues with him trying to use the software that came with his network adapter. When I removed what came with the card, Windows recognized it on its own and it was then stable.

Just an idea.