I can think of a couple reasons why this happens; some kind of hardware glitch, or a bad module getting loaded.
Boot VL and start looking through the logs. You need to log in as root. Go to /var/log/ and examine the files "messages", "debug" and "syslog", see if there may be some clues. Look for a time index that would correspond with the last time your machine locked up with the card installed.
Another quick and dirty test is to try a different PCI slot for the card.