Try another ROM for sure - recall a similar issue with some when I flashed over a few of these. Now that it is partially going, I recall there is a utility that lets you flash the card under OS X, without having to plug it back into your PC.

EDIT: that would be Graphicellerator:

http://thomas.perrier.name/graphiccelerator.html

... you can also adjust GPU clocks and RAM clocks, quite a handy tool to have on hand!

JB