Just to be clear and to complete the previous answers. It happens to me the same problem, i.e., the fft GUI is blocked (shadowed in grey color and not responsive). In my case, the problem occurs when pressing the "Peak Hold value" when using a Q6600 @ 2.40GHz with 4 GB RAM and NVIDIA GeForce 7300 SE/7200 GS. I checked it with UN210 and USRP1. Also, under a slower computer (core duo and 2 gb ram) the problem is increased. However, in both cases the CPU use is not at 100 %.
If i execute this command line the problem seems to dissapear (checked in the faster computer and with XCRV daughterboard).