CUDA tasks freeze

Hi, I have GTX 680 card.

First I installed ubuntu 12.04 and Cuda Toolkit 5.5 with Nvidia driver. All was fine, I did my scientific calculations. After some period of time I had to reinstall my system. I repeated all the previous steps and installation was OK (at least, no error massages was reported). But now, when I try to start a computing program, it freezes at the point when it has to detect and use GPU. This behaviour doesn’t depend on program: even CUDA samples freeze. The system is working and no error massages are produced, but the task starts from terminal and hangs. One of my 8 CPU cores is 100% loaded and huge amount of swap memory is used according to the top statistics.

I was afraid my card was broken so I booted from USB stick with default nvidia driver (normally, the image is generated by second ATI card). Graphics was OK and simple 2D and 3D tests was completed without fail.

What could be the reason? Is the problem with hardware or software?