Could be overheating or dodgy drivers.
What are the temps of the card? Use GPU-Z click here & HWMonitor click here to find out.
Did you try other driver versions? Archive is at click here (XP) or click here (Vista). Let me know if you're using 64-bit Windows.
Sometimes the only way to pinpoint such errors is by a process of elimination. Did they (the system builder I'm assuming) try with another working GTS250 graphics card?
What's the PSU make & model? G