USBs have a song limit of 2500 songs in our cars, if its more than that, it will probably freak out.
I have the same 8gb card, but only have around 600 songs, (more than enough IMO), and I dont have any problems.
All songs in the root, and only directory.
Is there anything else on the card? That could slow it down, and possibly cause problems too.
Also, depending on the software version of her module, there may be problems. Check out this thread for more info.
http://www.camaro5.com/forums/showthread.php?t=31235
The current version is CE, supposed to be in dealerships now.