Are you sure that just X freezes? Currently there seems to be a problem in the rt2x00 drivers that locks up the whole system. I have no problem with kernel 2.6.28 (which uses rt61pci driver ver. 2.2.1) but as soon as i try a newer kernel, the compat-wireless package or the backports package which all use ver 2.3.0 of the driver my machine starts experiencing lock-ups even when idling without any significant traffic. AFAIK F11 uses kernel 2.6.29 so the problem is likely in the driver.