Possible for a faulty 8800GT or a PSU. More likley the card as GPU-Z wont read clock values and PSU maybe because it won't stick to new values..?
http://forums.overclockers.co.uk/showthread.php?p=10408472#post10408472
It's looking like it's the PSU. Some dodgy thing that is just going to get worse with all these GT's getting bought all over the world. Antec's Customer Service is up for a busy Saturday morning it seems.