mouse/keyboard not responding under Xorg

[michael bane <michael.bane@manchester.ac.uk>]

I've not managed to define precisely when I get problems but I'm
noticing
the following symptoms and was wondering if some kind soul would help me
debug what's going on in order to get to a solution? Thanks, Michael

set-up: etch, gnome (eg gnome-core 2.14.3.6), metacity 2.14.5-4 and
Dell/Logitech USB optical mouse (seems to work fine in WinXP), with
nVidia graphics card/TwinView and 2 monitors (diff resolutions)

symptoms (occasional and not sure how to repeat)

a) double clicking won't select anything
b) in evolution, clicking on a new message or folder has no effect
c) the cursor (arrowhead) disappears - if I set prefs to show cursor by
depressing 'CNTL' I get the moving rectangles but there's no cursor
there)
d) focus doesn't move to new window (as it should and usually does)

I think the following is also a symptom but not 100% sure:
e) doing ALT-TAB to move between windows does nothing on very first
ALT-TAB

If it's the keyboard that's failed then using I also cannot get Gnome
menus (it selects the top level but nothing is shown) but can use the
mouse to open more new app/windows and then at some pt the keyboard
seems to be 'reset' such that it starts working again.

I've tried KDE and that also has the keyboard not responding (eg if I
try to switch desktops quickly then the keyboard stops responding) -
opening the control centre and changing options in there appears to
'reset' the keyboard.

Any thoughts either on what's going on or how to debug it to narrow
down the issues? I've asked on the Debian mailing list but am still
stuck. I've had this line in my xorg.conf re Mouse device:
	Option         "Protocol" "ImPS/2"
although it's a USB mouse I'm using, but commenting it out I still  
see the same issues

When the kyb "freezes" it is still alive since /dev/input/... shows  
characters (not sure how to do similar for mouse clicks)

Adding boot kernel option 'noapic' (as suggested on some fora) doesn't
help

So now I'm stuck as to how to solve this and how to narrow down  
potential causes so all tips welcome.

Thanks, M