keyboard is intermittently ignored

[Sam Steingold]

emacs cvs head on amd64:
every now and then the keyboard is ignored for a while, then everything 
is back to normal.
mouse works: I can select, paste, use menus (including undo), but the 
keyboard is silently ignored: I can type into xterm (actually, gnome 
terminal), thunderbird (like I am doing now), caps lock and num lock 
work fine - but emacs does not respond to any keypresses (including C-g 
and C-]). loadavg is 0.
I cannot figure out what triggers this behavior and why the keyboard 
input resumes (usually, after a few minutes of inactivity).
I am running under gdb, so if you tell me where to look, we can probably 
debug this problem together.
Thanks.
Sam.