I dont't understand when people say it's hard to reproduce. I have posted several times to the linux-mm list how to reproduce this problem. I can do it 100% of the time. My ubuntu laptop (currently running 11.10), unplug the power, put in a usb stick with FAT or NTFS, and start writing to it. As soon as the process has written about 1/4 to 1/2 of my physical memory, my browser starts randomly blocking. Happens every time.
My workaround is "watch -n 5 sync" when I copy files. Makes the problem go away completely.